to me, it depends on the time of year that the black foal is born. This filly was born Mar. 28th. Her dam is black and her sire is chestnut.
I have had foals that were born a greyish-cream body color that shed into a black, but I believe they were smokey black and/or maybe even grulla.
Usually the black foals I've seen are born a bluish-grey-black.
Towards June, they tend to be born quite black. Here is my mare that was born June 22 (she is now 6 and the dam of the filly above (this mare is out of a buckskin pinto mare and by a buckskin stallion):
just dried, you can see the silvery/creamy look to the tips of the hairs:
Here she is about 4-6 weeks later:
Around my place, all you need to do is breed ANY horse and it comes out black (just kidding!). Honestly, though, as long as they carry the genes for it, you can get black. Most likely to give you black are ones that are homozygous for the gene, and they themselves will be black. Bays and buckskins also carry the black gene (but modified by other genes). Even sorrels and palominos can have black foals.
Genetic testing can be done to determine a horse's color possibilities, but color should always be the last thing one worries about unless there are health issues associated with it (such as LWO).
Liz M.