0) be sure to have an alpha channel on the layer (Layer > Transparency > Add Alpha Channel)
1) use Colors > Color to Alpha... to make the blue full transparent
2) use Colors > Levels...
- select the Alpha channel in the Channel: dropdown list
- in the Output Levels, move the black triangle to the right
1) use Colors > Color to Alpha... to make the blue full transparent
2) use Colors > Levels...
- select the Alpha channel in the Channel: dropdown list
- in the Output Levels, move the black triangle to the right