Actually, the early third season uniforms were also made of wool gabardine, much like the final version. They switched from spandex after the second season because of complaints from Patrick Stewart and the rest of the cast on how the one-piece was uncomfortable. (They were made a size smaller to keep the smooth lines.)
Blackman was charged with making the uniforms more comfortable, while still maintaining that slim, fitted look. Which is why the early season three uniforms seemed as if they were also made of spandex. The seams and darts, in my opinion, were effective in maintaining that look, whilst still allowing the uniform to be a two-piece. They were changed to the blouser version with spandex belt because the early tunics rode up much more.
Wish they'd found another way to make the early versions work, because the seams and darts really gave the uniform more shape and looked much better on the men. The final version made the men look far bulky (or fatter) than they really were, particularly Jonathan Frakes.