If you've seen even just one flea plus the black dots you are talking about, you have fleas. The black dots are flea poop & eggs
. First, remove all her bedding, towels, etc and throw them in the wash (anything she sleeps with). Then vaccuum really really well (under the bed, etc) all carpets and hard floors.
Then, immediately wash the dogs with any dishwashing soap. You don't have to buy any flea shampoo or anything with pesticides- fleas will die with any soap. Make sure you start AT HER HEAD and work down, otherwise the fleas will run to her head to avoid the water and soap. After she's really dry, apply a topical flea treatment if you haven't already. This won't stop the fleas from biting her, but will stop their reproduction cycle.
Some people swear by MuleTeam Borax (a laundry booster) sprinkled in the carpets and along baseboards. I do this just to prevent fleas- but you can buy it at almost any grocery store (in the States anyway!). Sprinkle it on pretty heavy, then use a broom to sweep in into all the cracks and carpet fibers, then vaccuum it up. Supposedly this helps kill the fleas and their larvae and prevent them from reproducing.
If you've just seen this one flea, and this is not an ongoing problem, this should be enough to nip it in the bud. But fleas reproduce REALLY quickly, so you have to be quick! With the fosters I have coming and going (and almost every one has fleas!) I've never had a problem by following the directions above as soon as I see one. Good luck!