Funny you should say maroon... I used to have a maroon wool cooler with hunter green trim...for a bay. For the horse in question, I am thinking black or white would be the flashiest as a gift, and while I think white would be stunning, I agree it's just not practical! He also has four white socks
Another poster mentioned black making her horse look brown and I know what you mean - this horse however is very black and I think it will be awesome
There's another horse in the barn who is black who has a navy cooler which looks pretty great (gold trim!) but not as dressy as a black one would look, I think.
Also, I'm thinking no trim -- all say aye?
"I have never let my schooling interfere with my education." - Mark Twain
As others have said...no white. Caution with the black, personally I think it might end up not looking that exciting, depending on the fabric. Deep colors or jewel tones would be the best. Somewhere out there is a color wheel for horses and it tells you what colors look best on each color horse.
Aye to trim - AND piping! If you are putting black on a black horse the trim is what is going to make it pop. Otherwise, if the black matches the horse exactly (pretty unlikely) it's boring, and if it's slightly off it will look faded, or even worse, make the horse look faded. The trim is the frame, the piping is the mat. The picture will look best with both.
Any color will work, that's the beauty of black - but maroon trim with white (or metallic gold) (or both!) piping looks fabulous on a black. So does dark green.