Hi,
There are some interesting thoughts on improvising throughout this site but I remember posting a few ideas
here a while back where there was a good discussion on this very topic.
One of the main things though (as always) is listening to and transcribing recordings or little phrases that you like the sound of and using them as starting points for your own ideas. The great thing about the blues too is that because it is usually constructed from dominant 7th chords, there are just oodles of different scales and sounds you can play with over them: Mixolydian, Major/Minor Blues, Diminished, Whole Tone, Altered, Be Bop, Chromatic and various major and minor Pentatonics.
Are you comfortable with playing the chord changes (arpeggios) as well as using the Blues scale? If not this is also a good place to start. If you are playing a standard blues in D, start by playing the arpeggios of each chord and follow the changes. But then progress to harmonising the mixolydian scale as each chord plays. So on a D7 you could play arpeggios as follows:
D7 / Em7 / F#m7b5 / Gmaj7 / Am7 / Bm7 / Cmaj7
Listen to the sound each arpeggio makes in relation to the wrtten chord and they should take you off in a new direction. I particularly like starting on a Major 7th arpeggio, a tone below the key note of a dominant chord, it's got a nice flavour for me.
You can also think about starting each phrase a semitone away from a chord tone and then resolving it. Again in D you could play: D#-D, G-F#, Bb-A, C#-C. Create some exercises based on these ideas and gradually start working them into your solos, it will become second nature.
If you're already past these stages, forgive me - you will be looking for more professional help.
Good luck!