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The black spots you see are not the eggs, but "flea poop". The fleas bite the animal, suck the blood, and excrete digested blood. It looks black, but if you wet it, it turns blood red.
I swear by Frontline spray. Advantage doesn't work for Lady since it's not waterproof and I bathe her weekly. The Frontline spray works just as long as the spot on (3 months), but it also kills fleas instantly. It's much more econonical, too, since I can also use it on my cats. I just spray some on a cosmetic puff and rub it on their coats.