You can move the Pixel object's alpha to a Mask and then blur only the blue channel of the object:
1. Copy the Pixel object's alpha to a Mask above it.
2. Fill the Pixel object's alpha to make it completely opaque.
3. Disable editing of all Composite channels except Blue.
4. Blur the Pixel object.
5. Restore editing of all Composite channels.
Do not merge the Mask back into the blurred Pixel object. The Mask needs to be at the top of the layer stack when exporting the result to an image file, to ensure intact RGB values in the export.
