In case if market was whitelisted, earned rewards and then became blacklisted, then it will be not possible for users to claim already earned rewards anymore, because update_market function revert for non whitelisted markets.
Give ability for users to claim already accrued rewards for blacklisted market. Maybe you should remove require in the update_market function. As i guess that blacklisted market will have 0 weight in gauge. Then make sure sync_ledger checks that msg.sender is whitelisted market.
Any market can be blacklisted. This means that rewards will not accrue for it anymore. 
When market is blacklisted, then update_market is not called for it for last time(and already accrued rewards are not available for users as i described in QA-04). And those undistributed earned rewards are not tracked in any way to be distributed to other markets. Thus, those funds will just stuck.
You need to allow user to claim all funds(and when blacklisting you should call update_market for last time) or you need to track unclaimed funds and redistribute them other markets.
