i find the little moment with blaine's back turned so it's interesting - you could read it as part of the performance since he's said the name of his song and the music's started, but to me it seems far more like he's trying to get into the performance - that sway he starts to do to feel the rhythm.
plus... i don't know, the song's all full of and about wonder and jubilant, boyish optimism, which is Not what that moment with his back turned looks like. he actually looks pretty nervous to me! it's sort of hard for me to read the hands-on-mouth thing any other way. which is fascinating because blaine, up to this point has not been a nervous performer - at least where anyone else could see. it strikes me as very true to blaine ‘i act like i know what i’m doing but i don’t’ anderson doesn’t ever want to seem nervous in front of the other main cast. sure, in an audition, but in general - it’s just not something we normally see him be open about.
but this season fleshing him out more as a character in his own right (something that only gets furthered in season 4 because as man great Moments as blaine has in season 3, he doesn't really have a huge Arc) means the audience gets to see through the curtain a little with him. i think the nerves could and probably do account for two main things: new school, new environment, he's still figuring out how people will react to him and not everyone's been altogether welcoming, but also there's the clear conflict with him and kurt going for the same role and blaine not wanting to highlight his insecurities more. which makes it so fascinating to see him clearly be a little unsure of how he wants this to go, but still ultimately he seems to soothe himself by like... feeling the music in his body and then lighting up the stage like he just can't help it. because he can't.