Your horse is black.
There are two types of 'true' black- fading and non-fading. Both are black horses, and contrary to common belief, fading is not a sign of being heterozygous for black.
Some horses look black in winter, but still maintain a subtle lightening around the muzzle, eyes, and/or flanks. They may lighten significantly in the summer or remain relatively dark. These horses are not truly black, and are actually "brown" (a misleading term for those unfamiliar with it... it's a variation of the agouti gene- the same gene which causes bay) Your horse doesn't have any such lightening, and so is almost certainly genetically black.