In this regard, how did Blair get infected in the thing?
Scenario 2 - Blair was infected at some point by Norris or Palmer, when he was locked in the tool shed. This probably would have occurred during the hour-long blackout when a fuse was blown (probably by the Norris-Thing) and Fuchs died.
One may also ask, how does the thing Shapeshift? Before a Thing can create an imitation or shapeshift into one, it must first assimilate an organism, which it can do in two ways. Alternatively, the thing can create an imitation by simply injuring and/or infecting an organism, which will then allow the introduced thing cells to assimilate its host from within.
Simply so, what does the thing do to its victims?
The Thing probably absorbs the person's knowledge/experience when assimilating, and can imitate it perfectly as needed. It may be able to "recreate" a victim's brain when convenient, and just go on auto-pilot.
What was the Norwegian saying in the thing?
Get the hell away from that thing