I believe that Darnell is indeed Earl Jr.'s father, and that the DNA test did not pick it up because of something that the FBI did to Darnell during the time they were in the witness protection program. Remember, they said they were changing his blood type?
No one knows.....in the last episode it was found out that Earl wasn't Dadge's dad but Lil Chubby was. And then after Earl says that, Darnell barges in and says that Earl Jr. isn't his child either....then Joy says "Now lets just all calm down." then is says 'to be continued'. So no one really knows for sure....besides Dodge and Earl Jr. are about the same age and Darnell didn't meet Joy until, before her and Earl got a divorce.
crabman is earl jr father the toothbrushes used to verify dna were the same ones from when earl lived there only verifying earl is not the father of earl junior but is dodges father. crabman having his blood type changed and his chemical make up or dna are completely different things