I went to block a girl on SA. She got there first. Does that mean that I have lost the ability to block her? In other  words, is this a nuclear arms race style situation, where you really have to get your retaliation in first?

once you are blocked you can't block her. how can you block her as you are blocked and can't see her. The blocked can't block. LOL
If blocking is what you wanted to accomplish, she did it for you. But if wanted your ego satisfied by blocking her, you lost out. You can always create another profile, reach out to her and then block her LMAO.

As I feared. I wanted to block her not because there is a problem, but because she is my new found "regular".  With her blocking me, she can presumably see that I am still on SA, but I can't see what her level of activity is....   Would have preferred to reverse this situation!

Just create a new fake account, SA let's you in within seconds and you're browsing profiles as an anonymous new user.  Voila, instant anonymous lurking capability.  

this is SA--not a regular dating site. If she can't handle you being with other women, I think she's likely more trouble than she is worth. Just my 2c.

So what if you're on SA and she sees you?  Have you been stalking her to see if she is on SA?  

I mean, NSA man.