First, you need to select all the pixels. Because they're all one color you can select all the blue pixels:
(Other ways are
lClick on the layer's thumbnail. More complex images may need different selection methods like the Magic Wand tool, Quick Select tool, chroma key, or using channels to make a selection.)
Invert the selection so everything except the blue pixels are selected:
Make the selection smaller:
Border. Try different values here. Look at the moving, dotted lines around the selection area to visualize how much will be left after the next step.
delete key to erase the selected area from your layer