That area of the block seems to be a common area for blockage. When I rebuilt my engine I found the same problem as I prepped the block. I couldn't blow air through it. This was after scrubbing, pressure washing, and hot tanking. As George indicated above, poking at it should loosen the crud.
In my case I didn't think it would ever break loose and I wasn't going to rebuild a block like that. I tried all kinds of wire and had the best results with o/a welding rods. I even made up wire devices that I spun with a drill. I got a little hard black crud out each time and using compressed air helped. I worked at it and left it at least 20 times before it finally broke loose.
In my engine this same hard black crud had the water pump frozen up too. The truck sat for 24 years and I don't know if this was present the whole time or developed as the water evaporated/leaked out.
Hope it goes easier for you.
Chuck
