EWWWWW..the black stuff is flea crap. It's what they nourish the new hatchlings with. I was watching the grossest show on Animal Planet about "Extreme Biters", and fleas ranked 2nd? on the list of 10. They will consume far more blood than they need, but what they excrete goes to feed the millions of offspring they produce. YUCK! I find that if I wet my fingers with water first, and make a clean part through the fur, then the Advantage goes on easier. Helps to have a helper too. Revolution (I think) can be squirted in one spot along the shoulder blades, and absobes fast. Stock up, as it may be a problem for a while.