My best guess: Blue as in Leftist + Tick as in bloodsucker?
“blue tick” should be blue check I believe.
It’s a reference to a “VERIFIED” twitter account. I don’t use Twitter, but my understanding is that famous folks have verified accounts so you know it’s the actual person.
So there are 1,000 fake Trump twitter accounts, but only one real POTUS account with the check.
It’s a symbol of some status and acceptance on the site as having the check removed is a downgrade in some way. Needless to say, a bunch of conservatives got de-checked last year when big tech went after ALL conservatives on social media.