It removes some blue from the entire image, so most of the image looks slightly transparent:
Use a selection so that color-to-alpha is only applied where needed:
- using the fuzzy selection, select the background. Shift-click any closed areas (inside of 'O','P','D,'A', etc....) to add it
- Select>Grow by 2 pixels to include all the edge pixels in the selection
- Color>Color to alpha.
PS; You have strange artifacts around the big squares (the background isn't as uniform as it looks) so using C2A alone will leave some opacity in the background. A more complete approach can be:
- Make fuzzy selection as above
- Save selection to channel
- Select>Shrink by 2 pixels and [delete]. This insure that the background is completely clean
- Restore original selection from channel
- Select>Grow by 2px and C2A.
This more complete method is partially automated in the ofn-erase-background script.