I there any kind of dark chocolate that is not bitter and doesn't have sugar or artificial sweetener.
Depends on your taste in bitter, and how dark. If by bitter you mean 'not sweet', then you don't like dark chocolate. If you mean 'not nasty' then you can easily find palatable bars up to about 90% and above that with difficulty. Past about 75% cocoa, chocolate has to be of high quality to taste good while below 65% the sugar makes junk sorta palatable.
Just go to a shop which has a variety of brands and get an armful from 70-98% dark. Most of the latter are not eaten like food - dissolve a bit slowly like hard candy instead. A very few 98-100% can be eaten like the usual stuff.
Cocoa is naturally bitter; with rare exceptions its unpalatable without some sort of sweetener.
A few brands
Lindt. Cheap and available everyhere. 70, 85, 90 and 99%. Their 99% is a bargain but too sharp to eat it like food, see above. It was a terrific accent added into oatmeal or slowly tasted as above.
Vivani is a good bite for the $, but hard to find.
Vahlrona. Their bulk choclates seem to be better than their bars, at least that I've tried. I don't recall if they have an 85-95%.
ScharffenBerger Darkest below 100% is 82. I liked them better before they were bought by Hershey's. It's junk, does have a distinct flavor.
Green & Black 85% Very mild flavor.
Trader Joe's: Some good, some junk.
Some WholeFoods have a 92 which is tasty though not remotely sweet, don't recall the label. It's
not their store brand and not cheap, about $5/100g.
Watch the sugar and total food intake; you can restrict the sugar by using 90% or darker chocalate. Dunno about you, but my metabolism dropped about 80% between CF and being forced to stop working out.
I would caution against large amounts of chocolate as a permanent component of one's diet. Too dense, theobromine will keep you awake if eaten to excess and below the darkest grades the sugar can add up. It might be a worthwhile experiment but set an end date on it. Small amounts - less than one ounce a day - are no concern.