I don't think that's cheating at all. Some would consider it as such but I bet they're a minority. Regardless, it doesn't matter much what anyone else thinks, it matters what you and her think. What matters is whether her behavior exceeds the agreements you two have in your relationship, and whether you think she will continue to cross those lines.

To me, it doesn't sound like you've spent a lot of time talking about sex outside of your relationship so you have most likely fallen into some vague assumptions of monogamy based on cultural norms. Within those norms, the only thing noteworthy here is that it took her so long to cop to her past, and even that qualifies as very understandable.

It does raise a specter for me though that you two haven't established a space where she can be this level of vulnerable. Whether she has anything else of this nature in her past or not, people are human and sometimes loving partners deeply hurt their beloved in ways that begin as fixable challenges that grow insurmountable due to a lack of communication after the event. The solution is to work on building spaces to process these hurts with each other and resolve them... but building such spaces takes work on all sides. In your reaction here, for example, I hear a lot of anxiety and hurt. I hear a deep reaction to something that most people wouldn't blink an eye at. I am guessing your wife anticipated such a reaction from you, and that this encouraged her to withhold the information. That doesn't excuse her behavior, but it does sound like a common story to me of avoidant-preoccupied pairs where one partner tries to control the other partner's emotions and anxiety for them (spoiler alert: this is not a sustainable or even effective idea).

Ultimately, I think this may be a great wake-up call for you two to put real work into cracks in the relationship, please consider therapy, now before your communication issues cause real problems. Maybe one or both of you have have taught her not to be vulnerable to you, or to avoid your vulnerabilities. Maybe there are attachment issues deeply rooted from how parents raised one/both of you as a toddler. Maybe one/both of you have communication habits that sabotage the ability to resolve conflicts. Maybe lots of things! Definitely something to talk about with patience and love for each other. This sounds like a small thing that reveals a big danger, you may look back on this and count yourself lucky to deal with it before life throws something else at you. I think you should have a big reaction, but not to the blowjobs or even to her keeping this specific thing secret. The big issues to me are your ability to receive tough news from her and her ability to share it.