How old floor plans? To what scale? What size scanner?
To trace old plans, for good results you need the originals to be large scale working drawings. Then, you scan them to a good resolution (150 to 400 DPI) This usually creates image files that are larger than the maximum of 1024 by 1024 pixels supported by SketchUp (the blurriness you see is because SU is reducing all imported bitmaps to fit to this size). The size can be increased to a workable 2048 x 2048 or 4096 x 4096 if your graphics card supports it by turning on the “Use maximum texture size” option in Window menu>Preferences>OpenGL
If the original drawing has dimensions in place, you could just use SketchUp to “draw by numbers” without a raster underlay. The best option is to have proper measured drawings.