I have found that "blister" is most likely to happen when the steel has been repeatedly heated too high ( and held there too long), quenched, and then re-heated and re-quenched multiple times. I speculate that the quenching cracks the scale ( no harm to the blade, just the scale on the decarb) into little islands. Then on the next heat the decarb grows more along the cracks, eroding around the scale pieces in the hot oxidizing atmosphere. This ends up with the look of a dried lakebed or "bubbles". Many folks check the edge with a file and think the blade did not harden. Grind the edge back a bit and you will find hard steel. After tempering, grind/sand down the bevels to expose the hard steel everywhere.
Now, as to your problem with a warped blade:
When you quench the blade, the structure of the steel is austenite. This is really soft - almost like rubber.
As the temp drops rapidly past the pearlite nose, at 1000F, the structure stays austenite ( supercooled austenite). It is still rubbery.
When it cools to 400F, it starts to convert to martensite ... which is very brittle and hard.
You temper the martensite twice to relieve some of the brittleness and hardness.
During the time between 1000F and 400F the blade can be straightened by hand, put on a wooden board and pounded on with a wooden mallet, stuck in a slot in a 2X4 camped in a vise and twisted, ....etc. It won't break, and will take little pressure to straighten any warp ot twist. This is the time to do as much starightenong as possible. You will feel the blade suddenly start to stiffen as it reaches 400F. Quit any straightening immediately. After The second temper, and while still at the tempering temperature ( around 400F) you can try to straighten smaller twists and warps. Any attempt at room temp straightening will likely end badly.
Except for a severe warp and twist situation, or an incomplete hardening, there is seldom a need to re-heat the blade to straighten it. If you do have to straighten and re-quench a blade, it needs to be normalized first. Heat to around 1650F and let cool to black ( some folks quench each heat), repeat at around 1500F, and again at 1350F and let cool to black, then cool in water. The blade should be given a close check for any warp and straightened. Then you can re-do the hardening quench.
