Two questions
1) whats the difference between keldeo and resolute keldeo?
2) why is diggersby in bl?
Keldeo changes to Resolute form when you teach it Secret Sword, and changes back to normal form when it forgets Secret Sword. However, I believe it's possible to get a non-Resolute Keldeo that knows Secret Sword by doing some tricky thing involving trading back and forth between BW and BW2. For this reason, non-Resolute Keldeo is allowed to know Secret Sword on Showdown. If you try to use a Resolute Keldeo that does not know Secret Sword on Showdown, it will automatically make you use a normal Keldeo instead. The difference between the two forms is purely cosmetic (they have the same stats, ability, etc).
From a competitive perspective, you should always use normal Keldeo instead of Resolute Keldeo, because if you use Resolute Keldeo your opponent knows for sure that it will have Secret Sword, whereas if you use normal Keldeo you tell them nothing. However, virtually all Keldeos that get used do know Secret Sword (and so should yours, because it's a great move), so in practice it doesn't really matter.