How the pitch of a blade must change along the length so that the blade does not stall?
The question should really read, &quot;how the pitch must change along the length to keep lift distribution reasonably constant while respecting stall considerations&quot; or something like that.  Rotor blades twist nose down going outboard, like propellers but with less twist, to account for the higher velocity toward the tip, so that the slower moving root end is doing a decent amount of work making lift.
This tends to make the root end operate closer to stall AOA than the tips.  Whether it actually stalls or not is a more complex function of blade forward velocity (rpm) and vertical circulation velocity down through the rotor disc, whether forward airspeed is involved, whether you want to optimize for hovering or forward flight, and so on.
You're going to have to do some reading.  I found an ASE post from a couple of years ago by @Koyovis, relating to a question about blade twist, that links to a great information source, a book at Google Books that you can read online, called  Helicopter Aerodynamics Vol. 1 by Ray Prouty, who wrote a column for Rotor &amp; Wing for many years.  You should be able to assemble the information you need from that.