I also think that the user should get warned immediately when they try to add a blacklisted synonym. This feature was actually already suggested to the team last year, but I guess their priorities are currently somewhere else.
And in a different thread, they said:
In another thread, you have said that you plan to write a userscript yourself. So maybe you would also like to implement this functionality as a userscript? In that case, you could use this other script as a basis. It already uses wkof, you would just have to modify it to show the blacklist entries. You would have to filter the current item’s data.auxiliary_meanings
for the elements with type === "blacklist"
. (In case you want the script, but don’t want to implement it yourself, I can also modify it and post it here)