Working with layers - a headstart of a beginner...

[denzjos]

Use the 'Select by Colour Tool', set Threshold 25, set the 'Select by Colour' mode to add (second icon from the left below the tekst 'Select by Colour) and click on a few blue dots (dark blue, light blue,... what you want). Then 'Select / Grow' set 2 pixels and copy the result to a new layer and make a new white layer (or the colour you like) below the 'dots' layer.