By Janette G from Paradise, CA
I'm gonna say it's a blueberry bee. This picture best matches the coloration you describe. A picture can be found at: http://en.wikipedia.org/wiki/Osmia_ribifloris I hope this helps!
It might be a Mason bee, http://en.wikipedia.org/wiki/Mason_bee
My initial thought was a sweat bee. But if you are concerned catch one (dead-but recognizable or alive) and take it to your local agricultural office. It may have a different name but it should be listed in your phone book near other local govt offices. Another choice would be to take it to a state park if there is one in your area. Those rangers are pretty knowledgable about most of the critters in their state.
Without seeing a picture, my guess would be a that it is a blue bottle fly. We have them around here and they are harmless. Here is a link that tells about the blue bottle fly.
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