Hi,
I'm presently working on an M1025 (desert version) and I wondered about the very same thing. I did some research on the topic and there is no one simple answer to your question. On most pictures I found the chassis was almost completely covered with dirt or mud obscuring the real colour, however in some cases the frame was semi-gloss black as you've mentioned. Some of the HMMWVs presently in use are painted with their camo colour on the underside too, but in most cases the chassis is black and looks just like in a regular vehicle: dusty, rusty, with traces of oil and grease and with paint chipping on the panels. One thing that shouldn't be forgotten is that some of the moving parts (e.g. on the drive shaft) are in contact with each other while working and bare metal is clearly visible in some places...
I hope it isn't all gibberish
I will try to support my words with some photos as soon as I get back home from work.
Cheers