I can empathize with you because I'm exactly the same way. Doesn't matter if it's a "harmless black snake." They give me the heebee-jeebees and I have to avert my eyes if one is on TV etc. Did you see this HomeTalk thread? http://www.hometalk.com/1545982/snakes Unfortunately, moth balls seemed to be the most common suggestion but they are poisonous so you need to be careful if you have pets or small children that might ingest them.
By the way, we are in good company. DId you know the 2nd most common phobia is snakes? Spiders are #1. I'll deal with spiders any day of the week. Just keep the snakes away from me--far, far away, please! (Have you noticed people who do not have the phobia just don't understand it?)
Thanks, Shari, I did not know mothballs did the trick.