Black Mare & Sorrel Overo Stallion?

Yup. Would have to test his agouti to know for sure. Could also test the mare to see if she is homozygous for black or not. If she is, it means she can only ever have black based foals, if she is carrying red, she can produce red based foals too.

If the mare has a chestnut grandparent does that mean that she would be heterozygous? Or can she still be homozygous?

She could still be homozygous. Her parent that has a chestnut parent would be heterozygous, but she may have gotten the black from that parent rather than the red.

I see. Thank you for your knowledge!