However, for the most part, Eagles in space were composite shots. I don't have an image to put up, but if you get an opportunity to pop in an episode, space shots of Eagles and other crafts usually are in a part of the screen with no stars. There could be a starless path or the bottom third, say, of the screen would have no stars and the ship would be in that area. This way no stars would show through the shadowed portions of the models.
That sounds like the same kind of in-camera compositing done by Filmation's Space Academy
. Rather than using an optical printer and mattes to combine separate strips of film, which would result in loss of resolution, they'd film one image element, then rewind the exposed film in the camera, then double-expose the next element onto the same length of film, and so on until they had a final, first-generation composite shot. They used the same technique of leaving gaps in the starfield for the ships flying through the frame.
So that's not really the same as the bluescreen process ST used, because there are no mattes involved -- otherwise you wouldn't need the gap in the starscape to avoid image bleedthrough, since that's what the mattes are for. Unless what you're saying is that they created mattes using pure light/shadow contrast rather than a bluescreen, in which case the shadowed portions of the ship would just be missing from the image. That seems unlikely, though.