first u must learn your scales then impro comes easy. Most leads are NOT made up on the spot.
I think the foundation to soloing is to know the scales and when to use it appropriately. For Blues, you'll have to know the Pentatonic scales (but it's not limited to that). This will be the protocol to the solos. Also, I've been taught to use some arpeggiated chord tones to jazz up the solo (I am terrible at watching out for chord changes so I'm pretty clueless when it comes to that.). However, knowledge of the scales and arpeggios will only get you so far. You'll have to take it up a notch by using that knowledge to "musically speak" if you know what I mean. Also, with that comes formulating your own style and tone. Just have a listen to guys like B.B. King, Eric Clapton and Stevie Ray Vaughan. They can all play the same twelve-bar Blues progression yet sound so different! I hope this answers your question!
Learn the blues scale, of course. Learn your majors, minors, augmented and diminished chords. They help build blues chords and into soloing. Know your sharps and flats and how sharps and\or flats are in the key signature. Stevie Ray Vaughan, Eric Clapton, John Mayer, B.B. King, Jimi Hendrix, Jimmy Page, all of them use the standard blues format but in different ways of their own.
Learn your scales. Inside and out. Learn basic music theory and how the scales apply to harmony and chord structures. learn about voice leading and tastefullness.
When you have learned it - in your fingers, in your mind, and in your ear ....
I mean REALLY LEARNED it inside and out ....
Forget it. Let it all go and your fingers will know what they need. Your ear will know how to hear, and hopefully your brain will know how to switch off.
Then you will be able to play not scales or riffs, butsounds.
Of all the rules of music theory, the only one that really counts in the end is "If it sounds good, it *IS* good" and that can be a subjective thing.
There are no "wrong" notes. There is only tension and release. Learn where your heart wants tension, and when your heart wants it to go to more tension, or to release.