Hello Daniela,
It is hard to say what is causing FSL to display a white blob since I am not familiar with it. Can you tell what values fslview reports when click at some voxel? Perhaps some scaling value is getting ignored. Consider using: 3dcalc -datum float -a 'YOUR_DATASET.nii' -prefix YOUR_DATASET.f.nii and try again with the new float dataset.
cheers,
Ziad