“The reason to not wear denim is because it was invented by Levi Strauss for men to wear in mines, and therefore it is kli gever.”
That may be true, but that would apply to pants, not necessarily to skirts.
Nevertheless, one can not deny the fact that a denim skirt and a plain black skirt (same size, same length) are equally attractive.
Or, put it a different way; a young man wearing a tight denim jean will catch *more* attention than tight black slacks, IMHO.