BD - just so you know, you didn't puss out. Avoiding arguments and keeping peace in your home, sometimes means temporarily letting your wife hear what she wants to hear. You're at a place where you need to find words that will allow you to present your new understanding of this matter in a way that she can accept. She doesn't have to believe it herself- but she has to accept it - and as long as there is not a pressing medical issue at the very moment, then the acceptance could be allowed to come gradually.
"It was just talking about how if my son needed a blood transfusion I would no longer be able to keep quiet."
If this ever happened, you would likely actually have the right to keep quiet. (HIPPA) That is, nobody has to know what medical procedures you and trained medical staff decide. Instead of saying you won't keep quiet - why not point out that, over and over the WT has emphasized how medical decisions are a personal and private matter. While you have no intention of promoting any particular medical procedure to other JWs, What you personally choose will remain between you, the doctors, and God.
Another thing that might help is to look at the various articles where the WT has changed their belief on certain medical uses of blood, i.e. fractions, autologous, etc. Note the wording - it never says "It is now permitted" - rather it says " Some have felt their consciences would allow [xyz fraction, etc.]". This means that prior to the article being written making previously forbidden procedures a conscience matter (i.e. Acceptable) - "SOME" went ahead and accepted the forbidden fractions or whatever. Does that make sense? Basically the articles have stated that even though certain procedures were forbidden by WT policy - some went ahead with them according to their conscience. Were they condemned? No - actually the WT eventually brought it out that IT WAS A CONSCIENCE MATTER and presented it as new light so to speak.
So when things are nice and calm between you and your wife, and you can build up to a non-antagonistic conversation on her part and yours - ask her how is it some felt their conscience would allow things that the previous policy had forbidden? How was it they were not DF'd or reproved - but rather left alone and eventually used as an example on how it would be ok to view certain procedures?
Anyway - there are a lot of ways to approach this topic and it's very important to do so. But just remind her that husbands and wives should be allowed to have different consciences on matters like fractions, etc. If a spouse demanded that they have the same conscience on such matters and felt otherwise they shouldn't be married, that spouse certainly would not be respecting the sanctity of marriage. Once, perhaps after time has passed, she truly has accepted your stance on conscience and fractions, then she should also be willing to accept your understanding that a fraction might include RBCs, and plasma since the your trained conscience can't find the Bible-based definition for fractions to exclude such.
Anyway, pm me if you want to discuss this more.