So here's a use for blockchain
Everyone is always touting a bunch of stupid stuff for it like inventory or accounting, where there are already perfectly good solutions. There aren't very many business needs for a tamperproof ledger where neither side trusts the other, but between government and the public? Oh boy, now there's a place where something that fills in the huge lack of trust between the public and their government would really be useful!
If someone could figure out a way so that adding a 3rd "ghost user" to a conversation required interaction with a blockchain such that it left a record, then there would be a way to track when authorities did that, to count the how many times it was done and verify a court order was properly made in each case.
Parts of the court order, like who was targeted could/would be redacted of course, but making the blockchain public would allow the public to trust that the capability wasn't being misused for dragnet searches or by stalkers who have access to this as part of their job. It would require cooperation from the provider, but assuming it was automated to provide the ghost user when the court order appeared in the blockchain, there would be an up to the minute view of how much surveillance your government was performing. A smart electorate would demand a limit on how many times it could be used a year - sort of like having a cell plan that isn't unlimited it would force you to budget a bit and not use the capability frivolously.
I think many people recognize that there is some role for LAWFUL intercept in certain cases. What we're worried about is Big Brother type spying on everything, rather than only targeting terrorists, child molesters etc. which we KNOW will happen if they could get the "backdoor" they seek. If you have a ledger that shows every instance because there's no way to get access to those encrypted comms without leaving an entry behind - and no way to erase the entry - then you can have solely lacking but totally necessary "but verify" part of trusting your government.
This doesn't address other objections like "but the bad guys will just use something else" which may be true in some cases so you can't get them all, but it has been true ever since the first policeman that some crooks are too smart to get caught. No solution will be perfect, but I'd rather have something like this than have them blackmail companies into giving them backdoors without public knowledge. It already happened once in the US, just because we caught them thanks to Snowden, doesn't mean they wouldn't try it again.