A reader seeks advice on shopping for a wardrobe staple. Now that theater and travel are starting to open up, I realize that I need a basic pair of black pants — something between jeans and wearing a dress — but the choices are many and confusing. I would like a classic pair I can wear with flats or booties with a low heel, washable and in a style that will last for years. Skinny or wide-legged? Belt or not? Help! — Rosemary, Pleasantville, N.Y. The perfect pair of black pants is, like the perfect white T-shirt, one of fashion's holy grails. It floats, seemingly just out of reach in our imaginations, the answer to all our wardrobe ills if only we could find them. The perfect pair of black pants, after all, can take you pretty much anywhere. Toss a sweatshirt on top, sneakers on your feet and stroll down the street to buy milk or walk the dog. Add a blazer or a dressier shirt and some boots and go to the office. Throw on a tuxedo or a sequined top and high heels and attend a black-t...