What format are the input rasters? Ensure that the input rasters have same number of bands and same bit depth.
For Landsat 8 image inputs, you may choose 16-bit Unsigned as the Pixel Type.
Alternatively, you may also keep the output in a TIFF format (*.tif).
Here are a few reasons for getting a blank raster. If one of the rasters is having null values or zero values and you have selected the null raster as "First" in mosaic raster. or Second reason is a different pixel type in a different raster i.e. one raster is 8 bit unsigned and 16 bit unsigned.
Just select the mosaic operator properly.