It can be done. The question is though, do you want it done with an immersion type "hot blue" or with a decent wipe on "cold blue", such as Oxpho Blue?
If you choose the immersion type hot blue, just do the whole barrel.
If you choose the wipe on cold blue, warm the barrel up with boiling water and apply the coats until the portion you want to blue, matches the rest of the barrel color.