It's FOR SURE not a yellow, so whoever said that needs to look up the anal fin ray count that yellows normally have (higher than a black, usually higher than a brown). And the other characteristics aren't indicative of a yellow either (they have a rounded tail fin and white or light colored chin barbels).
It's either a brown or a black, but it does not 100% fit the characteristics of a brown IMO (though yes, browns can be unmottled, so that's not a species indicator).
For educational purposes let me post the fish with some characteristics highlighted:
Black/dark colored chin barbels that are only white at the base. This is indicative of either black or brown, does not distinguish either species. Does indicate the fish is not a yellow though.
End of caudal fin is somewhat indented. This is a characteristic of both browns and blacks, but does indicate it is not a yellow.
19 to 20 anal fin rays. Rarely do browns have such a low fin ray count, but it is possible. Yellows have a higher ray count, so this indicates it is not one.
Wide white colored line on BODY near base of tail. VERY indicative of a black bullhead. I don't think I have seen this on a brown. When they have a wide lightish colored line it is generally ON the tail fin at the base.
The only other way I can think to elminate one species from possibility is to know where the fish is caught and know whether either one is not present in that water source.