Wow Gymshark. I am sorry for what you are going through here. It does sound like you are frustrated and angry, which is to be expected. You are not alone. Your mom is never going to change, so the only thing you can control is your response to her.
I had an argument with my BPD mother she seems to say the same things resorts to nasty insults, threats trying to get my 10 year old little sister to Side with her
Your mom is triangulating here. Are you familiar with the Karpman Drama Triangle?
https://www.lynneforrest.com/articles/2008/06/the-faces-of-victim/https://bpdfamily.com/message_board/index.php?topic=108440.0she said im a cunt she's been mad ate for three days now
So I'm sorry she's using this language. It's inappropriate, but demonstrates she's emotionally immature. It's kind of like a young child that calls another child names because they haven't learned to self-manage their social and emotional skills. Do you think this fits your mom?
Do you know if your mom was abused as a child? This could explain low emotional
skills, as development can sometimes halt or slow after abuse or a traumatic event.
I usually ignore her don't engage walk away etc.
Good for you.

As you know, it's best to not engage. The person who is advising you otherwise - how informed or expert are they? People who don't understand BPD may offer advice, but that doesn't mean it should be listened to. I would suggest you stay with your strategy of not engaging. The more you engage, the worse it could get.
Im a psychology major so sometimes ill pretend she's a future client of mine and use the :look I can see your upset and maybe when u calm down we can regroup and talk"?
So this is good, but one thing could be improved. It is still a blaming statement because the word "you" is used to tell her she is not calm. This is what she is reacting to, because her disorder makes it unable for her to accept any responsibility. Therefore the statement is still invalidating to her. If you substitute the word "you" with the word "we", you may get a more favourable result. So you could say " It looks like you are feeling upset. (validating). That must be very hard mom (validating). Let's all take some time to get calm, and then we can regroup and talk if that is helpful. (presents a solution, and gives her a choice).
If she continues to rail on you, that is the time to say "I have to go now" and head to the bathroom, since you can't really leave the house with Covid 19. But maybe she'll stop because she so's surprised by your response. That could work. If she can't stop, it means her emotions have escalated too far. If that happens, just stay with support and empathy. Don't move to truth until her emotion is stable.
https://bpdfamily.com/message_board/index.php?topic=143695.0;all "Since I buy the foo in the house can u help and meet me halfway or just not eat my food and get your own"?
So BPD's typically have poor boundaries. I get your point here, and this might "check" a person who is not disordered. But it won't check a BPD, instead it will inflame it, which is what happened, but was not your intent.
It sounds like you are doing the grocery shopping for both you and your mom, but you each pay for your own food. Have I got that right? Here's another way of phrasing it to your mom:
"I am doing the grocery shopping for both of us (fact). I am happy to do this during Covid 19 to keep you safe. (fact). We each pay for our own food (fact). I feel disappointed that you are eating some of my food, because then that food is not available for me. Could you please eat just your food? (I statement). I will be happy to buy those things for you to use if you put them on your grocery list (offers a solution)." That states the problem, sets a boundary in an affirming way, and offers a solution. It is non-confrontational, and non inflammatory.
she snapped and then resorted to black mailing me and threatening my place to live
Yes - BPD's are high conflict. It's so hard to be around all the time, right?
she's a PLEASE READing nightmare to live with
She sounds like it.
im a grown adult with a child myself she tries to tell me what yo. do and I shut it down and say "oh stop while you ahead that ship came and went don't try and parent me now"
I get your need to say something since you are living under the same roof, but I would probably stop after the "ship came and went part". The last part just gives her something to engage with, and the drama will continue, which will increase your stress level further.
I got sober about 6 month ago

I am having to fully deal with the emotions and not numb them
So I'm reading between the lines here, but it sounds like you have done a HUGE amount of work, both in getting sober, and in analysing the BPD relationship situation with your mom. Good for you
she throws stuff in my face about my past eventho im sober. She is an alcoholic as well but still uses she drinks everyday
Just curious - how does she feel about your getting sober?
idk I guess im looking for new tactics to handle fights and or defuse them.
Good for you Gymshark. This is kinda where I was last summer/fall with my mom. I was emotionally exhausted, and you also say you are emotionally drained. I reached the point where I accepted my mom was never gonna change, so any changes had to come from within me, and how I responded to her. I just wasn't prepared to go on feeling the way I was feeling for the rest of my life. I'm guessing that's maybe where you are at? Does that sound about right?