Is the Mtl file required with the Obj? I have written a parser for an .obj/.mtl files. 

I was wondering: Is it possible to define a model without an mtl file?

If this is possible, what are the default material values?

I have spend a decent amount of time reading up on the formats of .obj and .mtl files. I have yet to see anything that talks about how to handle the case when an .obj does not reference an .mtl file.
You mean with Blender? 

The Blender obj exporter let you "write materials" or not, i.e. create the .mtl file or not (by default yes, uncheck it before confirm exporting).

And you can import in Blender a .obj file even without a .mtl

But I don't know if I answered you.