cdnum,
I did not mean to put the stamp in a chemical solution but only slightly warm water from the faucet. This would clean any dirt off and perhaps the red marks were dirt or other small pieces of paper still attached to the stamp.
I took your image and turned it on it's side and darkened the exposure to get the following image.

The stamp seems dirty and this may be just dirt but also could be mould (mold) otherwise known as rust or toning. I am not sure as the picture is not a scan so it is harder to tell. Or 3D.
However, even if it is, the red marks can be seen from this angle to be some kind of circular cancel but very hard to read.
So, registered cancels are sometimes red, yes, you are correct, but the state of this cancel (cannot read) and the change of colour of the other cancel (black to blue) makes me think that some chemical reaction has happened to this stamp before now.
You have a good guess at the fact that other cancels are sometimes blue but not like this. They would be a darker blue, not washed out like this, and blue is not used for a postmark in this era of stamps.
Perhaps someone was trying to wash the red cancel off. Perhaps the red cancel was a fiscal cancel of some sort and this would have lowered the value of the stamp a lot so there was an attempt to clean this off of he stamp but whatever it was that they tried to use reacted with the other cancel and perhaps the stamp also.
Do you have a photo of the back of the stamp? And a photo of the front showing the stamp laying upon a dark surface, with all the perforations showing please?
Please take the above statements with kind consideration. They are meant as guesses at to what possibly has happened to the stamp, not as fact. I am not expert at all on the Kangaroos (nor any stamp) but I have seen (and purchased) stamps that have been chemically altered for some reason or by accident. Colours change. Inks can react to different chemicals or even oxidize sometimes.
The ideal way is to examine the stamp itself, not a scan or photo, as then one can see things when light is reflected at different angles, and shadows tell a story also, but it needs to be taken as a whole not just a flat scan or photo.