Is my dress too 'dressy' for semi-formal?

I'm not sure it will look good with black heels if it's turquoise, unless the broach has a hint of black in it. You could wear a black handbag, though, to match the shoes. I think the actual dress is good for a semi-formal. Some plain jewelry might dress it up because it would be two plains. If you have any (don't go crazy), try some with pizazz that will attract the attention away from your dress.