These are my guesses on why abel is blonde
1st
Either "Eve is lilith" or "Eve looks just like lilith" theories are canon
Or
2nd
Lucifer and Eve had an affair making abel lucifer bio kid
Honestly, by way things are going with the writing, the insane amount of cheating, and how it is handled in both shows (especially Helluva)
I feel like it is going to be a mixture of both theories that are gonna be canon
Where lucifer had an affair with Eve because he probably saw lilith in her and felt pity for her being adam wife
And abel is born out of the affair, and Eve never told lucifer. Rasing abel as adam son, adam of course knowing abel is not his, is probably the reason why he didn't like abel (in the leaks lute revealed that adam didn't care much for abel)
Then the show is going to make a plot twist of "abel is lucifer son! How could you have ever known??!?"
Making even more sympathy for lucifer character. Also, to villianize Adam even more without considering the possibility of making him a complex character.
For Eve honestly I'm little worry fans going to villianzie her. Fans are probably going to call her horrible for not telling lucifer he has a kid or something. And how it her fault for the entire mess.
Again, If Vivziepop wants to retell the characters or redesign them in her own way then that's perfectly fine by me but maybe don't make them look thin af or look like tumblr sexyman bait.