When you're juust ready to start doing a task and then suddenly you get a mental block which makes it impossible for you to do said task. You get unfocused and frustrated.
The phrase comes from the expression "Locked in", which in modern slang means "Focused" (its antonym).
I was about to do my homework, but I blocked in and after that, I just couldn't do anything. Now it's midnight. I wasted the whole day. I'm sad :(
The tendency to back in a car, so that the license plate is not visible from the road, due to possible legal problems with the plate, (or lack of plate).
v. Black In: To only be lucid in parts of specified period of time.
Blacking In: parts of the evening when one comes out of a black out for a brief period of lucidity, and your brain begins to record and process information again. Usually followed by another period of blacking out.
So suddenly I blacked in, and I was in this hotel room with 2 naked chicks. I looked in the mirror and i said to my brain "Please, please, PLEASE!!!, DO NOT black out again!!!"