I use sand as a fill over whatever base I had - mostly limestone rock. My sand is black, too, but not sticky. I've never used Milorganite, but I have used a lot of ordinary corn meal and CGM on occasion. I would not think CGM or Milorganite would make sticky soil.

If you mean sticky like old gum, the organics shouldn't be doing that (but I guess if you've had a lot of rain it might be a bit sticky from all the happy little bacteria in there).

If you mean sticks together better than sand does, forming a bit of a clod when you compress it, the organics can be causing that and probably are.

Either way, it's good to hear the grass is thriving!