The blue background is actually fairly variable, so it is best replaced by something else to ease the processing.
- Color-pick the light blue
- Edit the FG color in the FG color editor, in HSL mode, reduce the L channel (luminosity) slightly, the idea being to obtain a color that is just darker than any blue in the image, but only just so.
- Bucket-fill the whole image with that color after setting the BF tool to Darken only mode (so in practice this will make the whole backrgound this slightly darker shade of blue paint (darked than the source image) but leave the text (black) and the text edges (dark blue) unchnaged). You can check the result with the Pointer dialog (values should be constant over the background) or with the Histogram (in RGB mode, a very sharp spike in each color)
- Color->Color to alpha and remove the background blue (pick the background color with the color picker in the C2A dialog)