Hello, my name is***** and I have over 20 years of experience as a veterinarian. I'm sorry to hear about your girl's excessive grooming, scratching, lethargy and shedding black specks everywhere.
The most common reason for these symptoms is a flea infestation and flea allergy dermatitis. She likely picked them up when exposed to the outdoor cats. Flea allergy dermatitis is horrible. In allergic cats it only takes one bite a month for them to itch like crazy. The black grit is flea dirt (flea feces).
Fleas prefer animals to people, so I'm not surprised you aren't being bitten. Animals are warmer, don't bathe often and have fur that is much easier to hide in. Her excessive grooming is likely removing/ingesting many of them, and thus making it harder too for you to see them. Most times people don't get bit until the population is overwhelming, or if there are no animals around when the fleas hatch out.
If you have other pets who aren't itchy they may not be allergic.
Many over the counter products have a problem with fleas being resistant and some can be down right dangerous. I never recommend any products that Hartz or Sargent's make. I recommend Advantage II or Frontline Plus topicals every 3 weeks or the new Seresto Flea and tick collar. All pets in the house must be treated as if you don't the nonallergic ones serve as a flea reservoir for the allergic one and you'll never solve your problem.
The quickest way to get things under control is to treat your home too. Only 5% of the flea population lives on your cat, the rest is in the environment. You need to vacuum your home throughly to remove as many eggs and larvae as possible. Concentrate on cracks and crevices, along baseboards, under furniture and areas where they like to sleep and eggs and larvae are at their highest numbers. Vacuum daily and throw away the bag.
I also recommend treating the home environment with a product that has an adulticide (kills adult fleas) as well as an insect growth regulator to stop egg and larvae from developing. A great product is Siphotrol II Plus Premise spray. I like it because it has an insect growth regulator as well as an adulticide. You can spray it under furniture and into cracks crevices and along baseboards where fleas and their offspring hide. It should be used every 2 weeks and no vacuuming for 2 to 3 days after using it to allow it to settle in.
Here is a link to show you the product: http://www.amazon.com/Siphotrol-Plus-Premise-Spray-16oz/dp/B000RGKX7M
The trouble with foggers is that they don't go under furniture or penetrate cracks and crevices so they just aren't effective where they need to be. And they end up in the air where they are nasal irritants for animals and people alike. Your other option is to have a professional exterminator treat your home and the outdoor area around your home. Ask for a guarantee and make sure that the products they use don't contain organophosphates and do have an adulticide and an insect growth regulator.
The key with flea treatment is consistency, not stopping too quickly, and using effective products.
To control allergic symptoms from the flea bites and get her more comfortable you can give either:
1) Benadryl (diphenhydramine only, the combination products with decongestants and acetaminophen are toxic to cats) at a dose of 1mg to 2mg per pound every 8 hours. That's a half of 25mg tablet every 8 hours for a cat that is 8 to 15 pounds.
2) Chlorpheniramine at 4mg twice daily.
3) Zyrtec (Cetirizine hydrochloride) at a dose of 5 mg per cat given orally every 24 hours. Make sure it is NOT the formulation with a decongestant (such as Zyrtec-D) because cats cannot tolerate decongestants.
Combined with high doses of omega-3 fatty acids antihistamines work to relieve her itchiness. I like 3V caps or Derm Caps. I recommend a dose based upon the EPA portion (eicosapentanoic acid) of the supplement as if we do that the rest of the supplement will be properly balanced. Give her 20mg of EPA per pound of body weight per day. For example an 8 pound cat could take 160mg of EPA per day.
Antihistamines and omega-3 fatty acids work synergistically, better together than either one alone. These should help reduce the itch. Be aware that antihistamines can cause drowsiness or hyperactivity which should resolve with continued use.
Other parasites like Cheyletiella or sarcoptic mange are possible causes of itchy skin as well.
For now I would start with continued flea control, an antihistamine and omega 3 fatty acids. If she's not significantly better in 2 weeks then it's time for a veterinary examination to look further for problems.
Please let me know if you have further questions by replying to this post.