I figure that it should be somewhat more casual because

1 - It's for a lesser-known performer (but the venue has hosted some internationally acclaimed artists like perlman and yo-yo ma).

2 - it's in the afternoon (v. the evening).

I was thinking "business causal" should do the trick. Do you agree?

Since it's a matinee, I agree that you would not wear a gown, but business casual is probably not quite dressy enough. Business casual makes me think of suits- either pants or skirt - and that won't do. Don't dis the performer just because he has not yet "made it" to the same heights as Yo-Yo. Wear a dark pallet, tea length dress and add a little bling in maybe the belt or as a necklace. You don't need to go all out for a -do, but your hair should not be loose. Either pulled back with a clip or a simple bun should do.

Be classy.

Stick to a styish black-metallic dress. Bring along a silver clutch and put silver barretes in hair or curl depending on length. Silver is in this Fall so don't resist!!!

Elegance and comfort are key. I would suggest wearing a nice classy black number that shows that you still care about your art. Too casual could mean that you just don't care enough about the gig.