The Catskill Aqueduct is one of three aqueduct systems that supply water to NYC. I would guess that "blow-off" refers to a valve that allows water to escape the pipeline if the pressure in the system exceeds a certain threshold.
Day 49
Catskill water blow-off
February 17th, 2012
The Catskills are hundreds of miles away from the city and, yet, there on the street is a reference to them. Yes, I know that this is a reference to the water tunnel(s) into NYC from that region, but the reference conjures a clash of cultural impressions.
Thanks for your walk and making familiar to many more people the local sights of the City.