Cobalt (CO) is the format used for 3D models in the program of that name from Ashlar-Vellum, which is a commercial product sold as a 3D modeling program for designers, engineers, and inventors. The format does not appear to be documented online. Other Ashlar-Vellum 3D modeling software with similar formats is Argon (AR) and Xenon (XE). Cobalt was formerly Vellum Solids, using the VS file type.
The files (of all three programs, AR/CO/XE) start with the string
CADD-PRO. They are binary files.
A free viewer for these three formats can be downloaded in the publisher's site.
Some sample files (of all three formats) can be found with the viewer above when you install it. The files are in
C:\Program Files (x86)\Ashlar-Vellum\CobaltShare v9 SP0\Samples in the Windows version.