Literally, blips on a radar screen are the little spots of light making a 'blip' noise as they move slowly about-- they are the airplanes coming and going at an airport, the submarines closing in on the battleship, the alien spacecraft approaching the Millenium Falcon with evil intent. However, it could be an idiom now, so we (or I, at least) need some context; alternatively, you could try to see how this literal meaning might be applied analogously to a metaphoric one.
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