Posting audio from a concert. Is it illegal?

Last summer I got the chance to see my favorite band live. During the concert, the lead singer did an acoustic version of one of their songs and it was beautiful. I recorded a video of it on my iPhone and I was thinking about stripping the audio from it and posting it as an audio post on Tumblr. I run a fan blog for said band, so I know my followers would really like to hear it. Is this illegal, though? It's obviously not a high-quality recording, and it would only be one song. Could I get in trouble for it? Thanks!

Added (1). It's obvious, but I thought I would just add: I obviously wouldn't be selling it, making a profit off of it or let other people download it. I just thought it would be a cool thing for people to hear.

Unless you own the copyright to the song or have permission to use it from the copyright holder yes, it"s illegal.

Tony's answer is correct, assuming there's a copyright. Most groups really appreciate the fact that they have fans. The best thing to do is to ask them. Most bands have some kind of contact info on their website. If you post it, disable the comments section to keep out the haters.