It's not clear whether your problem is due to the type of grass or the
environment (fertilizer, pH balance, etc). Have you put down any
fertilizers recently? If not, it may simply be lack of fertilizer. I
also have contractors mix and I thought they were a bit too wide.
Also, fertilizer should make the grass grow denser, which will hide
the dirt. It's always a good idea to have the soil sample tested by an
independent lab (not a lawn service company).