like doing licks , solos , improvisations using blues backing track?

What do I need to learn ? (notes on the fretboard) etc.

Do those people know how each note sounds like or they just randomly play notes when they are improvising?
listen to tons and tons of blues music.

Most blues improv are always played on the minor pentatonic/blues scale. Learn those. Learn songs , 12 bar blues etc.
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There are numerous video lessons you can watch,also try to cover as many blues songs as you can.
If you could reason with religious people,there would be no religious people.
I cannot stress how important it is to learn how to do the rhythm guitar section first. Learn a basic 12 bars blues pattern, and maybe a couple others in different keys. (A, E and C are probably best to learn). This will really help you get that feeling for the blues quickly, especially combined with a lot of time spent listening to the blues.

Learn your scales ALL ACROSS THE NECK! Don't just stick to the easy positions, and definitely try to learn some long ascending and descending patterns which cover all of the neck. And brush up on your bending and legato techniques.

Now it's time to learn from some of the masters. You'll find absolutely hundreds of licks all across the internet. Try and learn some from the most famous guitarists (Clapton, BB King, T bone Walker, Freddie King, Robert Johnson and some John Lee Hooker). Don't just look at what notes they're playing, the biggest thing is the notes they AREN'T playing. Blues lead guitar isn't about playing constantly, its the phrasing and pausing that give it that edge. Sure there are fast parts, but you don't want a solo compromised of guitar wankery (not in blues anyway lol).

Once you have a good feel for the music, you can begin making your own licks, at this point they should come pretty naturally from all the work you've done before, and if you learnt your scales inside out, you should have no problems making a good sounding lick or solo.

Learn some blues intros, turnarounds and endings then find a friend, one play rhythm and the other lead for a 12 bar blues progression then do a turnaround and swap to the other playing rhythm. You can keep this going for as long as you want, but definitely try to swap between a couple of keys, don't just stick to the same one