The undated double ring cancels were among the earliest of postmarks used in North Borneo. There were 3 post offices which were issued with this type of handstamps. They were Sandakan, Kudat and Gaya.
What has been described as blue is more likely indigo as seen in this fresh example. Indigo ink pads were sometimes used as an alternative after it was found that the red (or more accurately orange red) ink which was used washed off rather easily or yielded a cancellation that was too faint.
The black double ring cancellation was from Singapore which was used on an adjacent Straits stamp for overseas mail. Otherwise we have SG1 with a very nice Sandakan 14 bar cancellation.
It has also been suggested that the treasurer general of the time, Alexander Cook, was responsible for this marking. Again, the reasons behind it remain obscure. We also have the black Singapore double ringer to mark overseas mail.
Ps I have added an update to my previous post on Airmail hand stamp. You can see it by clicking here.