Normalize Mask

Objects painted with transparent materials do not create fully selected masks. If your source object is 50% transparent then your mask will also be 50% transparent. When the blur is applied it does not replace the original pixels, it is blended into them. If this is not what you wish to occur then select normalize mask to normalize your mask selection to 100%.

Mask from an object which is 50% transparent
Normalized mask from same object
General Mask Parameters