Barbacks flit from bar station to bar station, refilling garnishes, polishing glasses, stacking napkins, hauling kegs, bringing glassware back from the dishwasher, helping punch orders into the bar POS, and chatting amiably with customers who have questions.
Safest bet would be black slacks and a white button down shirt. If the bar has a theme, you're better off waiting to see what the dress code might be. I would dress for it just like any other job interview. Business casual or a bit more dressy.