I think the first three seasons were shot on 16mm rather than 35mm, except for "Entity," in which the crew experimented with HD video. Seasons 4-7 were shot in 35mm, as were the two post-cancellation movies. Seasons 8-10 were shot in HD video.
Now, 16mm can look quite good in HD -- it certainly has more resolution than standard definition video -- but it won't look as good as 35mm footage. And that doesn't cover the show's visual effects, which I presume were not rendered in HD.
Given the waning popularity of the franchise, I'm not sure a Blu-Ray release of the flagship series is in the cards.