There are no 727s in that photograph of Paul Howe [et al], they are various different configurations of R0723.
"True" black anodizing didn't really show up on Colt rifles until well into the M4 era, 7xx series carbines from the factory would have been one hue or another of gray, ranging from "Preban gray" to "Charcoal gray."
Also, refurbished/rearsenaled weapons were often coated with a flat black coating, and some are reported to have been sprayed with black engine paint.
For the most part, however, I suspect most photographs where they appear black have more to do with color saturation/contrast/etc.
Referencing that photograph again, if you look at the darker carbines, the black polymer pistol grips and handguards still contrast with the receivers, and there's definite contrast with the black vests/kneepads/other equipment.
~Augee