It sounds like she knows that you are interested, but that could just be your hyper-awareness of it that makes me think so.
Either way, you value her friendship, so just keep cool and casual. If she calls, great, if not, then go to the concert, have a fantastic time, and then when you're talking about it afterwards be sure to just casually mention to her that you had an extra ticket and you didn't want it to go to waste, so you tried to give it to her but....
Eric and I met in school, and we were friends first. I had an idea that he might like me, but I wasn't ready for anything so I just kept things on a friends level. Later on, it occured to me that everything I liked in a guy was right there, so I'm glad he stayed with it.