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Krishna's color is quoted from scripture to be like the hue of newly formed rain clouds. See Brahma Samhita 5.30: veṇuṁ kvaṇantam aravinda-dalāyatākṣam- barhāvataṁsam asitāmbuda-sundarāṅgam kandarpa-koṭi-kamanīya-viśeṣa-śobhaṁ govindam ādi-puruṣaṁ tam ahaṁ bhajāmi For the full word for word translation, see Bhaktivedanta VedaBase.(http://vedabase.com/en/bs/5/30) An overall translation is below: I worship Govinda, the primeval Lord, who is adept in playing on His flute, with blooming eyes like lotus petals with head decked with peacock's feather, with the figure of beauty tinged with the hue of blue clouds, and His unique loveliness charming millions of Cupids.

Krishna is not the only avatar/deity to be represented as a blue figure. He's accompanied in this by Rama and Kali to name just two. The blue color is a symbol. Both Rama and Krishna are considered to be shyama varna aka of 'dark color'. According to Amara-kosa: trisu shyamau harit-krishnau - the word shyama means harit (“green”) or krishna (“black or dark blue”). So, in terms of 'actual skin color', these folks were probably supposed to be very dark skinned (also something very very possible in the Indian subcontinent - possibly even more so than the wheatish complexion), with Krishna possibly supposed to have a blue undertone to skin. After all, the Jagganath representation of Krishna is portrayed as 'black' not 'blue'. To be extremely mundane, possibly, blue came to be used as a symbol because it's easier to portray blue skin in visual representations than really dark / black skin.

Everywhere Krishna is mentioned as black skinned coloured (Shyam/Krishan Varn). May be black is more associated with Nishachar/Rakchasa, so krishna is shown blue. But in many temples krishna murti are in  black colour.

There are no blue colour murti of Krishna as there is no blue colour natural stone. You will find only blue color pictures. May be getting natural blue colour was easier in good old days.