The blockchain wallet is one of the most secured wallet we have in the cryptosphere hence it would be really difficult for such an incident to occur. This is news to me, I have never heard or seen a reversible transaction without the involvement of the sender. Maybe you didn't explore the transactions on your wallet very well.
you can't be serious ? blockchain wallet is a web wallet
which makes it much more prone to different types of attacks than hardware or standalone wallets
if I were you OP I would check to see who could have had access to your account
you won't get your money back , sadly but at least you will find out if your machine has been compromised and avoid such incidents again
contact blockchain support for IP and device logs , change passwords to stronger ones , scan your pc for malware and be careful in the future
p.s. RBF transactions explanation can be found here:
https://en.bitcoin.it/wiki/Replace_by_feein short: if a replace-by-fee transaction is confirmed , it cannot be replaced by another one with a higher fee
so I would exclude this scenario