I think you're right, a tabby not a purebred but oh how lovely! Thank you for taking him or her in and giving them their forever home. What's their name?
Hi. Very striking cat with gorgeous eyes!! Most cats are considered Domestic (Short or Long Haired) and of no particular breed unless they have pedigree papers. Tabby is the label used to denote the basic pattern, with further pattern definers such as mackerel, classic, spotted, or ticked tabby. So, I think your cat is a DLH brown mackerel tabby. Brown is considered part of the black family in terms of coloring.
Just wanted to comment as a colony caretaker, can't help with what breed BUT his eyes look great, his fur looks great and his weight looks great, appears quite healthy..............Thank you for caring..............