About the black dress - if it is her favorite dress, I would say go with it. If you are not sure, this is really school specific. Some of the bigger schools say the LBD is perfect, other schools have in their recommended "what-to-wear" pamphlet to stay away from black or white because the girls in the chapter will very likely be wearing all black or all white for prefs. A color would make you stand out more, and it would help members remember you when making final invitation decisions.
Here is Auburn's very thorough "what-to-wear"
https://fp.auburn.edu/greek/schedule.htm
Alabama's and Texas A&M are similar --
http://www.uapanhellenic.com/index.p...d=14&Itemid=31
http://cpc.tamu.edu/node/58
But Georgia's is a little vague ?
http://panhellenic.uga.edu/fall-2013-recruitment.html
"What do I wear?
Casual skirts and dresses are appropriate for rounds one and three. Round two will be t-shirt day. All the participants will wear their recruitment t-shirt. Cool, lightweight fabrics are a necessity because of the hot weather. You will be walking a lot so comfortable footwear is essential. Heels are discouraged for Round 1-3. Nice dresses, skirts or pants that you would wear to a wedding or religious service are appropriate for Pref Round. Pref events are more formal, therefore many women choose to wear black dresses and heels although please note that this is not a requirement. The most important thing during all rounds is to make sure you feel comfortable."
I
think they are recommending an LBD for prefs.
If you have a nice black dress - you should still take it with you. You might need it later for rituals or a party.