A "blue-line stream" means that when you look at a topographical map, and there's a blue line, that designates a stream there. It usually means the stream is wet/moist most of the time.
In most locales you can build a pond on a blue-line stream, but you have to follow local, county, state and federal laws, often based on how large the pond will be.
Checking with the NRCS first is a good idea.
