Select on colour range/blurry colours?

[tmanni]

Try this:
1) Colors > Map > Color Exchange... to replace the light blue by white
- Pick the light blue as "From Color" and set the "To Color" to white.
- Set all thresholds to 1.000
2) Colors > Desaturate > Desaturate... to throw away any colors from the black text
3) Colors > Levels... to enforce the black
4) Colors > Color to Alpha... to make the white background transparent