As anyone who's worked with art will probably know, whenever an artdefine points to a nif that doesn't exist, Civ Renders it as a bright red hemisphere.

I'd like to know, is there any way to replace this default mesh? so that users of my mod would see something different as the default when a nif is missing.Is this red blob defined somewhere that I can change ?

I would guess, that it's not really a graphic, more a graphical curiosity.
If not, then it's sure hard-coded somewhere.

I think it can be changed as Warlords shipped with art files such as 'Default_Cavalry' which were the red blobs with a checkerboard pattern on them. Its likely defined in the dll though. I don't think it would be part of the graphics engine.

It's probably somewhere; I think the developers said that there was no hardcoding in the game

If it is hardcoded it would be in the EXE almost certainly, as most graphics are handled there. But even if hardcoded, the question is, WHAT is it hardcoded to point AT. Because then we could place a new NIF with the same name and directory path to replace it. So even if hardcoded as a reference, it ought to be replaced easily enough, but only if we can find it.

I've tried swapping out art/empty.nif and art/units/!errorunit/bear.nif, but neither seemed to have any effect. Though with what I know of NIFs I could easily have swapped things out incorrectly, as the NIF I tried to replace them with used more or less supporting pieces than each of those 2. (and in NifSkope empty really did look to be empty, and Bear looked like a bear... err, actually looked like a dog made of playdough by a 3 year old, but that is roughly a bear...)

I think it can be changed as Warlords shipped with art files such as 'Default_Cavalry' which were the red blobs with a checkerboard pattern on them. Its likely defined in the dll though. I don't think it would be part of the graphics engine.

I'm looking at the default cavalry. It appears to be a civil war era rifleman, on a red and black checkered horse. There are no blobs apparent there.