Just looked into this, and it is fairly similar to what I will do sometimes to relax before falling asleep.
Basically deep but not too deep breaths in and out and usually around the fifth breath there is a noticeable decrease in tension and I am suddenly very relaxed and able to fall asleep.
Didn't read very deeply into the two, so may be completely off the mark. But breathing exercises definitely do help reduce stress and tension, so I can see why they would help with blood pressure.
https://en.wikipedia.org/wiki/Kapalabhati
https://en.wikipedia.org/wiki/Bhastrika