Im not sure I think it depends, im sure some horses look black when they are actually brown, but my mustang mare Onyx was a sold black, however in the summer she had alil redish high light, very faint unless you where up close, she was the blackest horse off color supluments I have ever seen.
I have two horses that are black according to the AQHA. My gelding is black. He does not 'fade' in the sun. He bleaches in his tack area from /sweat salt. He is NOT brown in the winter. My mare is a smokey black - she can bleach out all over. I do not consider her to be a true black but AQHA does not have the smokey option as a color choice.
BLACK IS BLACK.
No, actually, genetically it's not. Regardless of the physical representation, my money would be on both your horses having the same basic genetic makeup for color. And "smokey black" means a black horse with a cream gene.
It's no different then having a sorrel, a chestnut, a liver chestnut, a flaxen chestnut, etc. Genetically, they are all "red".