There's no moral blackmailing.This is just factually incorrect as the decision was on Hillary vs Trump, which meant it was impossible to excuse the current administration.
It’s also not the case generally speaking and morally blackmailing people into voting your way isn’t democratic.
Do you think a third party candidate will win? I'll say no, they won't.
So a vote not for the two major candidates is a vote taken away from potentially electing a new President.
So whether you support the current President or not a vote for a third party is indirectly a vote for maintaining the status quo. Be that via a second term for the individual or the majority party etc. That's just how it works out, it takes a vote away at this moment in time.
The perceived benefit of two fingers up to the current system by voting third party is so wildly outweighed by the damage another four years of this administration could deliver to not only America but worldwide.