Minecraft – What happened to this creeper? It seems to have a blueish glow

There is a deep pit with 4 chests (which contain various items: iron and gold ingots, diamonds, emeralds, bones...) and a pressure plate in the middle. Underneath the pressure plate are 9 blocks of TNT which explode after some time if you step on it.

The player in this screenshot is in the place of the removed blue wool block. One piece of sandstone at the bottom has been removed by me to show the TNT. The room is well lit because there is a hole in the ceiling right above the blue block.

You can easily avoid the pressure plate if you dig out one of the adjacent blocks to the blue one (note that now the pit will be much darker), but the fall is still quite deadly and takes 4 hearts.

And here is a video that demonstrates this:

... It was catching on fire. On the right you can see the edge of a hibachi. The block above the gap between the two dirt blocks is then a valid ignition location which would be touching the axle. This would cause the gearbox, which is flammable, to catch fire.

So mystery solved, took quite a bit of testing and fighting with fire. Why can it go through solid blocks anyways :/

Anywho, thanks all whom helped me track this down!