I've got the same 21" Exmark. It came with the red blade that is the original you got. This blade bag wells and mulches decently. It does however have an annoying habit of leaving stringers when you cut at 2.5" or above. I've tried to cut with the front set 1/2" lower than the back but the problems still persists.
I contacted Exmark about the cutting issue. They responded by suggesting that I change blades to either their bagging blade or something called the extreme2 blade. Part numbers are: 108-3759 for the bagging blade and 108-0954 for the extreme 2 blade. I have no idea if either one would work.