What colour would a foal to a buckskin stallion and black mare be?

Palomino Stallion --eeCrcr (the two -- are the unknown alleles at the Agouti site)

Black Mare aaEe or aaEE (we will assume that the mare is not a smokey black which is a black horse carrying the creme gene). If either of the mare's parents is a chestnut her genetic makeup is aaEe which will simplify the choices.

If either of the stallion's parents was a bay/buckskin (or both parents were bay based ) he will have the possibility of carrying the A allele.

If the mare is aaEE no palomino or chestnut foal can be produced.

If the stallion is aaeeCrcr 50% chance the foal will be black and 50% chance it will be smokey black.

If the stallion is AaeeCrcr then there is a 25% chance the foal will be bay, 25% black, 25% smokey black, 25% buckskin.

If the stallion is AAeeCrcr 50% buckskin and 50% bay

If the mare is aaEe and the stallion is aaeeCr- the chances are 25% chestnut, 25% palomino, 25% black and 25% smokey black

If the mare is aaEe and the stallion is AaeeCrcr the chances are 25% chestnut

25% palomino, 12.5% black, 12.5% bay, 12.5% smokey black, 12.5% buckskin.

If the mare is aaEe and the stallion is AAee Crcr 25% palomino, 25% chestnut,

25% buckskin, 25% bay.

Lots of possibilities here, always with a 50% chance of the foal with the Creme dilution. Unfortunatley, if the stallion carries a black allele or two black alleles your Creme dilutes will have a high probability of being smokey black which is often hard to tell from black.