If it was something the seller wouldn’t have been comfortable doing due to their beliefs, morals or such, or if it was something they thought might go against Fiverr’s policies, they might have been afraid that you would still order their gig directly after they said no and blocked you to a void that.
There are sellers who have made that experience, telling someone they can’t or don’t want to work with them but that person still ordered. Then, the seller would have to cancel the order and might take a hit to their rating, ranking etc.
I hope that helps you understand this better, it was probably not anything you did wrong, unless you sent them something that would violate Fiverr’s terms, if a project looks as if it might be someone’s homework, for example, sellers will be wary.
Perhaps mention what your art project is about when you first contact a seller, that will save both sides time if they wouldn’t want to work with you for any reason.
I don’t think it’s common behaviour, unless it’s something that would violate Fiverr’s terms and/or the seller’s ethics.
I’ve had to say no to a couple of things because they were against Fiverr’s terms as well as I wouldn’t have wanted to do them anyway, and it’s safest to block the buyer then to not risk them ordering your gig anyway.
Sorry to hear this was your first experience anyway but if it was due to the seller’s own beliefs or ethics without being something that’s against Fiverr’s ToS, you shouldn’t have much trouble finding a seller, just perhaps tell them some more details before you’d send them the script.