Ok to add cellulose over blown fiberglass insulation

A layer of dust or dirt on top of your insualtion would be quite normal in a vented attic. If the dirt was inpregnated in the actual insulation, so that it was acting like a filter, then that would indicated air movement from one side of the insulation to the other. This would be seen most often around holes in the ceiling from light fixtures, plumbing/electrical or any areas open to air leakage from the interior to the attic. I think what you are seeing laying on top of your insulation is just plain old dust from outdoors entering through the venting. not a problem.

Cellulose (the typical, but not the only, blown in) is much less air-permeable than fiberglass. Some tests even show it works without a vapor barrier, though I certainly put one in anyway on my project using it. It also has an R-value that increases in the cold, and no seams.

Fibreglass batts are considerably more air-permeable, have seams, and have an R-value that reduces with cold temperatures - which is not great if insulating to reduce heating costs.

So: doing what you describe is both possible and better than doing nothing, but inferior to (and probably more expensive than) blowing in more cellulose. If you have a near-religious objection to more blown in, by all means level out the blown-in and add fibreglass batts, rather than not adding anything. If, (as seems unlikely from your comments) you'd like the best insulation job for your money, consider adding more blown in.