You're actually absolutely right. Someone has messed up the way the blur brush behaves on the newer builds of blender. Me and other fellow creators have noticed the issue and it's a huge inconvenience that no one else seems to pick up on. It's more noticeable with meshes that aren't plain or multiple meshes being rigged together like a coat with buttons.
It almost looks like someone took the smoothing modifier that changes the topology and made a brush out of it for the weight paint tool, because that's what it does, it sort of space out the vertices through weights. For example, if I have two lines forming a seam "====", if I use the blur tool to smooth the weights affecting it, instead of maintaing the shape of the seam, maintaing the correct space between the two lines, the blur weight paint brush will space out the two lines, almost like the smooth modifier would on the mesh topology, which I think is awfully retarded, I don't know who did that and why but I wish someone would fix that because it's annoying to go back to previous builds just to smooth weights.