Why is my booster fan pulling cold air from outside?
It would appear to be a poorly tuned fresh air vent (aka a "Make-up Air vent")

Blocking it completely is not a good choice.

Tuning it correctly is a good choice.

The intake should not be bringing in more air than the furnace blows out (far less, in general)

A page from 11 years ago...

http://www.nchh.org/Portals/0/Contents/Read_This_Before_You_Ventilate.pdf