Have seen (and worn) plenty of black dresses. I find there are no real "blending issues" despite not having stones because the dress material has a sheen to it, whereas the man's suit is more matte, so it's pretty clear that there are two people dancing together. Well-designed black dresses can be very tasteful and elegant, and if it suits the wearer's complexion, nothing wrong with wearing it. Just because a lady wears black doesn't automatically mean she wears it for a slimming effect. I think if a person dances well (applies to both the lady and the man) they will stand out on the floor no matter the colour of the dress.