The scarf idea is great! Be aware of your color choices. You do'nt want one sorority being offended if you wear another sororities color. I know it sounds petty, but it can be taken that way,.
As for the black dress, you will fit in quite nicely. Most of the PNMs wear black dresses now. It is simple and once you pledge, you have ot have one anyways.
Way to be one step ahead!