Sure, but in the case of blockchain.com, they claim they are entirely non-custodial and so do not have access to your accounts and do not know your private keys or addresses. If this is true, then they have no documents or evidence to provide and no information they can hand over. It will be your word against theirs as to whether or not the wallet ever contained the bitcoin in the first place.
They don't, actually. Their current Terms of Service explicitly say they act as a custodian of your assets, at least for tokens in a "Trading Account." They very much have a legal duty re those assets. And they have a legal presence and can be sued in the UK and US.p.s. Of course it helps if you've kept records of your own: screenshots, login information, etc.
People who are naive enough to use web wallets are almost certainly not keeping up to date logs or records of their own. If they were willing to go to all this effort, then they would just go to the far lesser effort of installing and using their own wallet.