well the way i started first improvising is just first listening to alot of jazz players or in your case rock/blues.
listen to alot of the musicians in that genre and see what you can hear. Also listen to a person whose played that music before and improvised.
Say im playing freddie freeloader by miles davis, ill listen to him and just learn a few techniques off the piano player(im a piano player) and then incorporate it into my improvisation along with my own skills.
Without a teacher i think thats the best you can do, listen to alot of that type of music and then you will come up with your own soloing and stuff.
Well you know the blues scale so thats good, muck around with it and see what licks and such you like, and then just incorporate it into your song.
edit: Improvisation is soloing, if you know what that is, its making up a "tune" on the spot if you like (i bet alot of the people here would smite me for saying that, im sorry!), like what the baroque harpsichord players did, they were given the chords but when it was there cadenza or whatever (im not good with classical technical words sorry

) they would make it up, that was until classical composers wrote down their solos for their soloists.)