I'd start by doing a lot of listening to the "greats" e.g. the 3 Kings, Chicago, Texas legends etc. I'd also invest in a decent blues bass book e.g. the one by Ed Friedland. This will give you a good overview of the main types of lines and riffs, as well as chord progressions and arrangements e.g. 8 bar blues, slow blues, Stormy Monday etc. I'd then apply the ideas from the books and play along to the CDs and try to pick up fresh ideas along the way from your listening.
I'm no expert obvioulsy, but I did used to play in a blues band and play it a lot as a means of escape from my function band.