Unless you are unquestionably important then harshly going against the grain as you wish to do is usually career suicide.
How should I deal with coworkers not respecting my blocking off time in my calendar for work?
Firstly, I work in an office and the only way to approach me is from the side of my cubicle and my screen is in view as you approach me.
Now I regularly have virtual meetings with people so I put on my headphones and talk.
It never ceases to amaze me the number of times I get approached, interrupted, and asked if I am busy. People will never pass up a chance to maintain tunnel vision and think that their need is important enough that they can turn off their situational awareness.
I regularly block off 1-3 hours of empty space in my work calendar to work undisturbed.
I hope you're away from your desk when you do this and don't answer emails and chats as if you were in a meeting yourself.
Very few other people I work with do this.
Right, because if everyone did this then it would be impossible to schedule around every person's "undisturbed" time.
Anyone can see anyone else's calendar, so my coworkers often add me to meetings during this blocked-off time.
They know you're available, so yeah.
Usually these meetings are scheduled 15 minutes before they begin so I only get notified while I'm in the middle of something.
This sounds cultural. Very rarely do I get invited to a meeting 15 minutes before start.
What's your industry/role? I'm a web developer.
If I tell them I'm busy then they get argumentative that I'm not actually in a meeting and that theirs is far more important.
Ask them to explain this urgent meeting to you: what's it about? who is going? what knowledge of yours do they need specifically?
You should aim to diffuse their urgency much like an off-color joke. If someone says something offensive and you say "I don't get it" then it typically quells the offender.
If I insist on not going they get really annoyed and complain to everyone else on the project that I'm not being a team player.
See above. Diffuse the urgency; make them explain it to you like you're 5.
My manager is good at multitasking while I'm not, and he says I should try to work on my stuff while being in these meetings. I don't think this will work for me. I should note that this also happened at my last job, so moving jobs isn't a great solution.
Oof, multitasking is dumb; this manager is wearing some seriously rose-colored glasses. They probably think delegating work to others and taking credit means they are multitasking; or they're simply never doing anything too intense.
Quite frankly, it sounds like they don't care about your problem because at the end of the day it is in fact, your problem. Worse yet, you are trying to make your problem into their problem; nobody appreciates that.
You have to learn how to do your work, attend an unexpected meeting, and continue your work once you're back at your desk. That's just how the majority of businesses run.