The whole idea around being blackmailed is that you are unable to communicate with town. However, being able to vote (non-anonymously) ruins this. You can communicate that you are blackmailed by spamming the vote button, and if you are a known TI town can ask you yes or no questions that you can answer by voting on certain people.
Removing the target's ability to vote would be too overpowered, so the solution is to make either everyone's votes anonymous or just make the blackmailed person's vote invisible. Making all votes anonymous kind of ruins scum-reading, so the second option is preferable. However, if you were *very* careful at counting votes, you could tell that someone that voted was blackmailed, and this would be unreplicatable by mafia. Realistically though, this is minor compared to the current flaws of blackmailing.