To me "meditation" is a vast sea of various techniques. Meditation isn't a thing and there are soooo many ways of doing it that just talking about whether meditation works or not is not very meaningful to me.
Some meditation ways or techniques work much better for me than others. In particular, "mindfulness" to be aware of the present moment throughout the day helps me checkin in and see when I am getting off track into mania, anxiety, or depression. Then, I have a chance to use other techniques such as CBT or journalling or art therapy or other techniques to cope with it or a specific form of meditation such as counting breaths.
On my own, I tend to drift a lot in meditating and lose direction (and intention). The past 2 weeks, I was fortunate to take a live, online mindfulness beginners class that was given because the Meditation Center was closed because of COVID. I learned a lot. Reading how to do it hasn't helped as much.
What works best for me in coping with BP and anxiety is a mindful concentration meditation of counting inhales and exhales 1-4 in time with slow breathing. The slow breathing helps me calm down. The counting gives my busy mind something to do rather than spouting even more thoughts.
The counting is a stronger anchor to the present than trying to focus on breathing one in many forms of meditation for me. My mind drifts and gets pulled away into my thinking if I try to focus only on my breathing. I usually have a very active mind that goes all over the place (especially when I am manic or anxious), and the teacher pointed out that for beginners in Zen, they may spend a year or more only doing counting meditation to try to train the mind to concentrate. Part of my difficulties with meditation have come because I tried to do too much and advance too fast and do techniques beyond what my busy mind can do now or perhaps may ever be able to do ...
I try practice throughout the day (and night) as well as during short sits. The benefits for me come in trying to do it throughout the day, not just in short sits and expect it to carry through the rest of the day. Others may have a different experience.
The past two nights, I have used the simple counting technique after I woke up to fall back to sleep again, and it worked well. I have slept better even with some physical pain than I have in a long while. Tonight I was at a small meeting, and I practiced it for a while and it helped. My problem will be remembering to do it, but daily practicing helps me make it more likely to remember. “Mindfulness isn’t difficult, we just need to remember to do it.” – Sharon Salzberg Can I do it 100% of the time--no, but it helps when I do, and I try to practice self-kindness when I forget.
Meditation for me is practicing and learning when works for me--the teacher for the class I took made the point that people differ and to be mindful of what works or not and to make adjustments.
Note I am not saying that meditation or mindful meditation will cure or treat all mental health conditions, especially severe ones. But it helps me cope with mine. It especially helps me cope and reduce my anxiety that sometimes triggers my BP and vice versa. Too often with mania, I like it. So I can be mindful that I am going into it, but choose to not try to nip it in the bud before it is out of control....
This is my experience, yours may be entirely different.