Even when you're an expert there's a chance of misidentification, but
take a look for yourself at a black rat snake pictu

http://www.snakesandfrogs.com/scra/snakes/images/bratabv.jpg

As I posted before, repair the defective vent with new screen or put in
a new replacement vent. If you give it some time it should leave on its
own looking for food, provided it has a way out.

Raajah wrote:

While working in the garden this afternoon I came across a 3-4ft black
snake. I do not know if it was just a garden snake or is poisonous.
Anyway, I tried to get it away from the vent which is about 6 inches above
ground leading to the crawl space. The snake went into through the vent
into the crawl space. Does anyone know hot to get it out and prevent
further intrusions. Is there a chance it was poisonous? Who can we call
for help?