The blockage phenomenon in aerodynamics usually refers to sonic blockage: as the flow reaches the speed of sound, there is a point where it cannot manage to go any faster unless the right conditions (in surrounding geometry, temperature and flow differential) are there. So essentially, the flow stops accelerating at the speed of sound, and this is referred to as sonic blockage.
In the sentence you quoted, the term "blockage" is misused as it refers more to the debit of air in a duct being mostly dictated by pressure differential between entry and exit, so you can't "add more air" without changing the pressure in the duct.
For thin and long ducts, there is also the fact that the skin friction on the walls of the duct locally slows down the air and creates something called a "boundary layer" in which the air progressively accelerates as you get further away from the wall. When the duct is very thin compared to its length, the boundary layer from one wall and from the opposite wall join up, so the whole mass of air going through the duct is slowed down, not just on the edges. This phenomenon is sometimes also referred to as blockage, but it's not the correct term for it.
Do take what I'm saying here with a small pinch of salt as most of this is translated from my knowledge of aerodynamics in French, and the technical terminology in English might vary a bit.