I've been trying to analyze this stamp with various tools and I finally found a method that highlights some oddities. First, it gets very green if the saturation is drastically increased. So the black ink does have a green tinge to begin with.
Next I finally found a way to highlight the areas that seem altered/cleaned/blurred by chemicals/etc. that I noticed along with srailkb (Ken?).

I can't tell you off hand what caused those altered areas, but they don't seem to be natural. I agree with others in that I just don't like this block's eye appeal. Also, top right Washington's left eye stands out so much that it is a bit creepy and draws my focus to it every time that I look at the block as a whole.
The next image is probably just me seeing stars/rings:
