Is black snake with one yellow line behind its head poisonous?

Not liklely. That sounds like a common Garter Snake. It's always errennious to consider a snake poisonous or nonpoisonous marking its external body coloration only.Since nonpoisonous varity could also bear same markings.Even juvenile species of same snake could be of different coloration as compairing to the adult ones.So to identify snakes perfectly one has to consider more specific markings like scale count,head shield type,belly scale type,presence of poison fang etc which would require more proffessional knowledge.Here to answer the question we would require more clues to explain the same.The clues given is just not sufficient.