"Black" or "Green" rifles refers to modern sporting rifles that cosmetically resemble firearms that the media would incorrectly describe as "assault" rifles.
Civilian legal 'military-style' sporting rifles are not select fire (cannot fire full-auto or burst) and by definition, cannot be properly or legally described as "assault" rifles.
The black and green terms refers to the most common colours or finishes found on these rifles. The original M-16 (which incidentally does meet the requirements of an assault rifle) was also nicknamed, "The Black Rifle." However, we firearms enthusiasts have co-opted the term for our own use, as it is relatively benign and print friendly.
In Canada, semi-auto sporting rifles such as the AR-15 and variants, Rob Arms XCR, SIG Classic Green/Black Special/Blue Star series of rifles (outwardly they resemble the Sig 550 series/family but aren't), Armalite AR-180 and a host of others all fall under this general, catch-all 'category' of "Black" & "Green" rifles.