coolien,
People seem to go around repeatedly pointing the "dark" J-11s as J-11Bs. They're not. The J-11B has a black nose like the J-10's and a bit fat on the buns. Also you will see it carrying Chinese missiles like PL-8, PL-11 or PL-12, not Russian types like R-73 or R-27.
The dark J-11s in my opinion, are J-11As. My theory is that these are made without Sukhoi supplied kits. Some of the parts are still supplied by the Russians but they're should be sourced directly from the manufacturers instead of going through Sukhoi, which can make the parts cheaper. But nonetheless, they offer nothing better or more than the standard grey J-11s whose airframe structural components are supplied from the kits. The J-11As are there to prove the worthiness of a completely Chinese made airframe before going to the next step, which is the J-11B.