So sorry for your loss OP. I hope you are looking after yourself. Your BF's parents have lost their child, and that is something most parents will never get over, BUT, their grief isn't a free pass to treat others like crap and blame others for their son's decision. Looking at the rest of the world, many parents lose their children to suicide, but plenty can see through the grief and do not blame others for a decision only their child had the ability to make.
Suicide is an action that is rarely talked about, so there is usually little or no verbal indication that they are planning it. A minute number of people may do it on impulse, but those cases are so rare, and the vast majority of suicides do give a few indicators that something is wrong. A few signs could be listening to sad music and saying that they missed a deceased relative. Or seeming distant for a long while. Simple things that we don't really notice and tend to pretty much keep under the radar IYKWIM.
Your BF's parents would still probably blame you even if you had a stupid, minor argument over what kind of car you were going to buy. As was rightly said, suicide is a big indication that the deceased was suffering from MH problems beforehand, and they were severe enough to drive the person to take their own life.
It's so much easier for his parents to blame yourself and the rest of the world, than to look at themselves and see that they missed the signs that indicated he needed professional help. Not many people want to reflect on their failures in life, especially when it comes to missing clear signs that meant a loved one/friend needed help and then losing them in this way.
You should let your BF's parents grieve, but if I was in your situation, I don't think I could ever speak to them again after this.
You sound like a lovely, kind, caring person, so let your actions speak volumes more than your BF's parent's words. Eventually people will see that they were wrong to treat you as they did.