A really dumb way to protest, a blank ballot just allows someone else to mark it for you. If you truly can't chose, then mark both candidates which would prevent someone else picking one or the other for you.
That may not happen, since European voting is actually more secure than ours (a lot of countries vote on-line) but the effect is the same...if you don’t vote for someone or some party, essentially you’re giving your vote to other people who do vote - not necessarily for the person you’d like.