Don't discount the stallion yet, he might well be homozygous. There is a lot more going on with pinto markings than meets the eye. There have been several tested cases where homozygous tobianos can be completely solid, for example. At the moment the theory seems to be that there is a 'white restriction' gene at play, but no one can find it yet.
Hypothetically - black to chestnut, without going into it too much, can give you Bay, Brown, Black and Chestnut. You won't get any dilutes (palomino, buckskin, smokey black, perlino or cremello) and you won't get Grey.
Black to buckskin - again, all four base colours. You can then get single dilutes - palomino, buckskin and smokey black. You still won't get grey. You also won't get any double dilutes - cremello or perlino.
That makes alot of sense now. Thank you!