While the other answers adequately touch on the matters regarding your raise, I'd like to address some other concerns you raised.

She's obviously playing the office politics game. And at the moment she's taken the lead.

Usually she just talks about her products (in which I helped her a lot) as if she did them all alone and they were perfectly done. Which isn't the truth. She also puts all the "easy work" that she doesn't want to do on me and keeps all the bigger jobs, but at the same time complains that I help way too less (everytime I'm done with one task I ask her for another one) and that she's way to overworked.

I'd strongly suggest trying to get some of the bigger jobs yourself, to get more visibility with your boss, and his superiors. One way to do this might be to stop taking her lead, and go to your boss directly, indicating you'd like some bigger projects to work on.

As others said, document everything you can, you ask her for more tasks, do it via email / some ticketing system. If she mentions being overworked, send her an email saying hey I heard you're bit overworked, I'd be glad to help out with 'some bigger project you'd like to work on'. If she does not reply, or keeps giving you the smaller stuff, go to your boss, saying you realized she's overworked but trying to do it all herself. That you tried asking her to divert some things to you but she didn't want to.

Now Stacy is on vacation and blocked me out of her mail-access. I am the one who does her unfinished work when she's on vacation and the other way around. The jobs she left me are a complete mess, and I can't get any details because she blocked her mails getting through to me. My coworkers complain to me that these jobs aren't done yet and I'm still in the prep phase, because I need to collect every detail and make sense out of it. I texted Stacy over Whatsapp, she read the messages yesterday (ticks are blue) but didn't answer me. I'm furious and stressed out right now.

If you can't take care of her running business while she's away, because she blocked access to information you need, you should make sure every person concerned knows about this.

How far you want to go in this is up to you, but here's some options

Alert your boss

let your boss know what is happening, because sooner or later, complaints will reach him about your 'bad' handling of the ongoing tasks while she was away.

I'd personally send him a quick email, mentioning you are responsible for taking over her ongoing tasks but can't. You might add that she deliberately blocked your access, or not, depends on how far you want to take it. How many hints you drop regarding the messy state of her ongoing tasks, is up to you.

By doing this, you ensure your boss at least knows why you can't do the things in due time. Depending on your technical setup he might also be able to get your IT department to grant you access anyway.

Alert your colleagues

Again how you do this is up to you, but I'd suggest trying not to be confrontational about it. Something like this should suffice.

There seems to be a mix-up between Stacy and me and I can't seem to access her mailbox to get more information on the matter at hand. Do you mind briefly walking me through it?

That way, you don't really put the blame on her (nor on you) but your colleague is in the loop on the situation.

You could go a lot more confrontational as well, saying that she seems to have blocked your access, but I do not advise doing so.

Things to do when she's back

Plan a meeting with her, to discuss the 'mishap' that occurred. We both know it's not a mishap, but if you word it like this, she still gets a way out without loosing face.

The advantage of not being confrontational is that it is a lot harder for her to shift blame. You're not pointing fingers, so if she starts doing it in response, you've got the moral high ground.

That's when I'd start pointing fingers though. Than you can start actually saying what you said here in the question, how she blocked you etc... How she is overworked but does not want to give you any of the bigger things on her plate...