Greetings,
Mr. bp. "Blowers on a gas boat blow air (and vapours) out not in." Of course they do but I think Mr. ps's point is that whilst running, during refueling, they also CAN draw vapours in through the passive vents.
I would NEVER even consider running the blower on our gas boat during refueling. I refer you to page #38 of the Transport Canada Safe boating guide where it is recommended that during refueling, among other steps, the boat should be closed up (hatches, ports etc.). I suspect US recommendations are the same.
Running a blower during refueling could easily draw explosive vapours into the engine compartment due to wind conditions and the fact that the "passive" air intakes (which CANNOT be closed up) might not necessarily be "...on the other side of the engine compartment..."