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POP/ROCK The Blasters: ''Non Fiction.'' (Warner Bros. 23818-1.) The Rockats: ''Make That Move.'' (RCA MFL 1-8507.) Though both these groups draw the bulk of their musical ideas from an earlier era of rock, they sound as different as they possibly can. What they share is a freshness, a tightness that is lacking in many top-40, videomaking specialists. And they also share - along with other groups like the Stray Cats and the Go Gos - a vision that looks back as well as ahead. The Rockats draw heavily from rockabilly, country-western, and other areas, but they sound, of the two groups, more contemporary, especially with the '80s-sounding keyboards by Danny B. Harvey. Apart from one song written by Marvin Gaye, all the rest were homemade - and they can certainly turn out a good tune. ''Go Cat Wild'' is one example, a classic-sounding rocker one could be tempted to flip up the volume on. Dibbs Preston's lead vocals have just the right amount of snarl to them. In an extended play album of strong cuts, surprisingly, ''Make That Move,'' the title song, is perhaps the least compelling. The Blasters with their latest LP, on the other hand, could slip comfortably into '50s radio. Torch songs, for instance, are incorporated with just the right mixture of cars and anachronisms. Most of the lyrics are by Dave Alvin. ''Long White Cadillac,'' dedicated to Hank Williams, begins the flip side on a somber note. The excellent lead singing by Phil Alvin narrates the ballads and propels each song with style and conviction.