Horse coloring is a very interesting subject
There are a number of colors that look like black (liver chestnut and brown/seal bay/dark bay are the most common), but aren't really, and then there are some black horses that don't look like it because they fade in the sun! Smoky black horses (black + cream gene) can be particularly sensitive to fading in the sun.
How long have you had this horse? Has he ever shown any color variations before? Do you know what color his parents were?