Bill Blass was in his Seventh Avenue showroom the other morning doing final fittings on his fall collection with Jerry Hall and several other models. "That looks pretty nice, babe," said Blass when Hall appeared in a gray tweed full coat over a plaid slim jacket and straight checked trousers. "Big coats are the big news," he pronounced. "So are sweaters and suits."

By the time the models had lined up this morning for the Blass ready-to-wear show, the designer had changed his tune. "Pants! Pants! That's what I like. And sweaters. They are the newest jackets for suits," he laughed, enjoying his own change of emphasis.

Women's Wear Daily had been to see Blass in the meantime and decided to focus on his pants and sweaters. "I'm agreeable to anything," said Blass, as he tugged at his already loosened tie. His press release notwithstanding, if smart money was on pants he could as easily hype them. Besides, it was all legit--there were plenty of pants in the collection.

Blass' show at the Pierre Hotel kicked off the final sprint of the six-week marathon that has kept buyers and press chasing after the last word in clothes for fall in Milan, Paris, London and now here. (For some of the hardier souls, the trek continues on to Tokyo next month.)

Blass also has changed his tune from the very snug fit that characterized many of his clothes last season. "Let's face it. Fitted clothes are hard to fit on a lot of customers," he said.

And so, at today's show--for an audience that included Nancy Kissinger, Barbara Walters, Jerome Zipkin, the European, Japanese, and American press, and, most important, the buyers--Blass showed that he has abandoned the tight cinched waistlines, peplums, boned bodices and confining skirts of last season for a silhouette that is mostly lean, but never tight. Several jackets and tunic coats are plumb-line straight, sweaters fit easily over the torso, and evening dresses simply trace the figure. Most daytime clothes brush the kneecap and black tie get-ups are either to the knee or full length. And everything is loaded with globs of chunky, junk jewelry.

Not only is Blass more comfortable with this easier style, which includes lots of sweaters, sweatery jackets for day and beaded and chenille sweaters for evening, but his customers obviously like the change. "I can't wait to get back into slacks and sweaters," said Kissinger, who told Blass exactly which sweaters, pants and evening clothes she wanted to order. "I adored the seven-eighths coat in purple and brown stripes over the brown suede skirt."

This season, American designers, never shy about keeping an eye on their European counterparts, picked up on a way of showing clothes that once characterized the European collections but in recent years has wisely been abandoned. Until recently, the fashion crowd had to traipse all over Paris and Milan to catch designer shows at galleries, circus halls, parks and tents; now those shows are held in a central location for the convenience of the audience and the models.

Blass moved his fashion show from Seventh Avenue to the Pierre Hotel several years ago for the convenience of the fancy customer crowd that attends his shows. And now other designers, who do not have enough room in their showrooms, have taken their acts all over town. Akira showed his clothes last Friday at a theater near Broadway (with Zipkin and Pat Buckley in the front row and Arlene Dahl a few rows behind them). Ronaldus Shamask used a runway set up in a cavernous Tenth Avenue sound studio (once used to film part of "Sophie's Choice") for what he billed as a "photo opportunity." Models went through their paces slowly to give the standing-room-only crowd a good chance to see his strong, clean-shaped coats with single, pointed lapels, jackets and capes with the same lapel treatment and lean sweater dresses and plisse' crepe separates. Geoffrey Beene has scheduled his showing in a television studio in Rockefeller Center (Studio 8H, originally created for Toscanini's broadcasts), complete with television monitors to zero in on the details of the clothes. And Ralph Lauren is using nothing short of the Seventh Regiment Armory to show off his wares. "People always complain that my shows are too crowded," said Lauren before his presentation. "No one should be able to say that this time."