Barca didn't try to block Neymar's whishes. You got it all wrong.
Barca tried Neymar to CONVINCE him to stay. As in convincing him that staying at Barca is better. Once he let the team know that he wants out, Valverde even released him from training to deal with PSG.
That's because Barca knew they can do nothing against the release clause. There is no release clause for Dembele. How are Barca even allowed to agree terms with a player who's got 4 years left on his contract and whose club is not interested in selling?
You think Dortmund didn't try to convince him to stay? How can someone so blindly follow a club and think everything they do is holy and everything that's not working out well for them is the definition of evil.