South Indian temple stone idols are normally black - some interesting facts!!

Huge Siva Lingam, pouring of milk. Thanjavur big temple.Temple, Travel and Sport
Narayana Perumal  moolavar Melkote, karnataka Temple, Travel and Sport
 ''The temple is the house of a God  who is to be physically present in it.  He does it in the guise of a ''cult object''- image or  symbol which depicts him in some way or other, He is the only divine power of whom such physical presence in the temple is necessary. His divine attendants are to surround him and this can remain virtual, .....A god's image whether sculpted or modeled in the round or in relief or else engraved, painted or just drawn has  to follow rigorous iconographic and  iconometric rules which can allow the precise identification of the concerned god...''  (vide: Bruno Dagens - The Indian temple). 

The cult objects or images of god are of two kinds -   ''definite'' and ''undefined''.  In the case of former, the good example is Shiva Lingam - this form is mostly found in the temples. The other definite symbol is Trident. In the case of Vishnu, the definite symbol is ''Slagrama'' ( a unique fossil called Ammonite found in the Himalayan terrain in Nepal) and the other one is Sangu (conch) and Chakra (Disc).  Shiva is often represented by undefined symbol designated as Natural Linga - self-existing  Svayambhu Linga. 

Once the image is consecrated by chanting certain specified mantras it becomes the presiding deity of that temple and divinity is attached to it. The shrine's sanctity increases as the time goes by;  the longer the time, the more sanctified it will become.  God's presence must be maintained by daily worship which is a must. Officiating priests, depending on the situations and puja protocols,  perform prayer in front of him  - invocation, offering of food, Deepa Aradhana, etc.The other puja ritual is anointing,  pouring of liquids, placing ornaments, etc on the deity. To withstand the rigours of regular pujas, anointing, etc in south Indian temples the deities are made of hard stone carefully selected by the Sthapathi - one who designs the temples.  

Regardless of regions and states, the main deity (Moolavar) or deities in the Hindu temples are normally kept in the center of the shrine called Garbagriha or Srikovil (sanctum) the most sacred place on the temple premises where only the temple priests are allowed. The center point of the Kumbam (cone shaped metal pot) atop the tower/ Gopuram is fixed in such a way  exactly its vertical axis  runs at the center point of the idol right below in the sanctum, for example the Shiva  Lingam. The belief is the cosmic energy in the surrounding area is, through the kumbam, focused right on the idol which absorbs it and radiates it as positive energy. In the sanctum one will also find  a metal idol or idols (made of an alloy of 5 metals). It serves dual purposes, in the case of Moolavar not made of hard stone, the daily abishekham or anointing is done on it. During festival times, the Utchavar idol(s) will be kept out side the sanctum in a mantap for darshan or taken out  in a procession. Hence Utcahavamurthy is often referred to as a procession deity.
Divine Black Stone Lord Ranganatha SwamyKanakkadara Divine Services

Lingam made of black stone Amazon

As to the question ''why is dark-colored stone used for idol making in south India?, the following are some  of the plausible reasons:

wooden idols of Puri Jaganath
01. Wooden idols, even if made from high quality choice wood, need replacement every 10 years plus because of lack of durability. After a long period it may lose its  strength become gently swollen. For example the wooden idols of Jaganath temple, Puri, Odisha  where all the three idols - Subadthra, Krishna and Balarama are made of Neem wood. They are replaced reverentially every 12 to 15 years time by freshly made new wooden idols.

02. In the case of wooden idols,  certain puja rituals are difficult  to perform as in South Indian temples  anointing with oil or liquids, applying sandal paste, etc are done on a regular basis. Normally floral alankaram/decoration is done regularly on wooden idols.

03. With some exceptions, invariably the idols in the sanctum in South Indian temples, be they Vishnu, Shivan, Ganapathy, Subramanyar or Amman/Thayyar,  are made of hard rock, preferably dark colored granite. The reason being the  major  part of South India is made of Archaean complex consisting mostly of metamorphic Gneissic rocks along with high temperature granite related rocks.   But the Sthapathis prefer granite related  dark colored rocks that are available in plenty in some parts of Tamil Nadu. to make idols. 

04. In south India, particularly in Shiva and Amman temples anointing with oil and washing of idol with liquids, water- sandal wood water, rose water, milk, etc are conducted  almost daily. Since dark colored granite has tough mineral content, rituals like abhishekam, anointing, etc will have little impact on the stone idols. 

05. As for Vishnu temples, such daily anointing or abhishekam of  idols is not followed, instead, yearly the temple priests conduct what is called ''Thailakaappu''. For this purpose the sanctum will be closed for several hours once a year during which time the priests will clean the idol and reverentially apply special  herbal oils made from  specific herbs, etc on the idol to retain the cosmic energy. As in Saivite temples, regular floral decorations, etc are done daily at Vishnu temples. 

06. This is the reason why ancient South Indian Hindu temples have centuries old stone idols in the sanctum and other shrines in the same temple. Mind you these idols were consecrated during the time of  ancient rulers who had built the temples. They are several hundred years old.

07. A fascinating fact in such ancient Hindu temples, the stone  idols have been there since their first consecration. None of them is replaced as of today.  

08. The high resistant minerals in the granite group are not susceptible to various chemically treated liquids poured on the idols during puja and the chemical reaction on the idols will be less.
09. Yet another advantage is dark color will contrast with the decorations done on the idol during certain puja . Frequently vibuthi (ash) and sandal paste are applied on the idol besides floral decoration, etc. With dark background, such decorations will enhance the beauty of the idol in the sanctum. 

10. Another plus point  in using dark stone  in the garbagriha where the main stone idol is enshrined is  it is hot inside  due to poor ventilation. The stone idol will absorb the liquids, etc poured on the it easily and will never get damaged.

11. In the sanctum only oil lamps are used and electric lights are rarely used. The belief is they won't interfere with energy-absorption. 
Saligrama stone (Ammomite fossil with whorls). Vishnu chakra. Devshoppe

12. A note-worthy fact is there are temples where not hard high temperature related granite stones are used to represent the main deity (Moolavar). Instead what is called Salagrama stone (highly compacted pretty old fossils with whorls called Ammonite) is used for making idols. For example at Sri Rama temple at Punnainallur close to the famous  Mariamman temple, Thanjavur, all the big dark colored Moolavar idols - Sri  Rama. his consort Sita, brother  Lakshmana and Sugreva  are made of Salagrama  donated by the Nepal Maharaja to the then Maratha ruler of Thanjavur a few centuries ago. The salagrama idols here are  one of a kind in the whole of India.

13. At temples where the Shiva Linga is Svayambu, it is either made of wood or loose soil (Putru - stuff from mole hill), normal puja rituals are done on the Utchavar kept in the sanctum. Because, the self-manifesting Linga  can not withstand the regular puja rituals that involves a lot of pouring of water, liquids, etc. 

14. In the making of stone idols or metal for the temples and other places of worship Agama Sastras are  strictly used as guide lines and the workers are expected to adhere to
a disciplined life.