Krishna’s skin color isn’t exactly described as definitely blue or black anywhere in the scriptures. Some scriptures give examples of this color as somewhat similar to the dark bluish rain cloud or the darkest sapphire. Most often they say ‘blackish’ or ‘of blue hue’, and further explain that the color of Krishna’s body is indeed called “shyam,”- this sort of color doesn’t exist in the material world. In The Bhagavad Gita, Lord Krishna in response to the request of Arjuna to show him His true form (Viswa Roopa), revealed His image as the cosmos and all that is in it. Subsequently the cosmos holds a color that from Earth seems dark black/blue (as the void, the vacuum of space- the ‘nothing’ that sustains everything). However, it’s impossible to create a representational pigment of this color because it doesn’t exist, so Hindu artists have preferred to portray the colour of God as dark or blue (in some cases black) to match the colors we see of open sky. Furthermore, black is the hue composed of all the existing colors combined together, so some consider black a more fitting color to represent the whole cosmos as the image of God.
Thanks for such an interesting question! I hope I’ve helped!
Jai Sri Krishna!