If it's what I think it is, get rid of them quickly. I have the same/similar in my yard and while they grow nice for a few years, before you know it, they'll be everywhere. They choked out all my perrenials. We dug out all our dirt, down 2 feet, bought all new dirt and still this 'flower' grows all over my yard. I can't seem to kill it or get it under control.

Delphinium?

not sure but possibly a bulb from south africa??? i think it's some sort of Scilla... ~ liz

how tall was it? what do the leaves look like? where is this at, hort zone? pretty rare i am stumped by flowers LOL...now i want to figure it out! ~ liz

A type of Camassia maybe?

Here is a link that might be useful: images Camassia

YUP! that's it! interesting it used to be thought related to Scilla's but DNA analysis found it wasn't but related to agave's. cool! ~ liz