Is there a way to block other E621 users? Like, say I have some risky art I don't want certain people to see, is there a way I can block those users so they can't view my posts or interact with my account in any way?

Sorry, but there is no way for you to stop others from viewing content you upload. However, what do you mean by "interact" with your account, as there is a way to prevent you from seeing them in the blacklist. You can use user: before the user's name, such as user:siral_exan, and check the appropriate options to blacklist them.

Nope. There isn't. I've read the site's "Help" section for years, and no such function has ever been mentioned.

Edit: NINJA'D! Curses!

you can mute people so you can't see stuff that they post but you can't stop people from seeing stuff you post.

if there's stuff that they don't want to see it's their own responsibly to blacklist stuff in their own settings.

Sadly nope. But if they complain about content they could blacklist, you can report them because its on the rules, cuz its their responsability to blacklist shit they don't want to see.
In the case someone harass you via dm you can tell them to stop messaging you, if they don't, you can report that too.