I just found it in a glossary added to the Strike Fighters 2: Vietnam Air and Ground Expansion Pack:

"Black SAM: a mysterious type of SAM that suddenly appeared during the summer of 1972 only
to disappear after a few months. These new "Black SAMs" were stouter than the familiar SA-2
Guideline, and were an ominous dark black in color. They also appeared to be resistant to
standard ECM jamming as well as being far more maneuverable than standard SA-2s. U.S. air
crews suspected these new "Black SAMs" to be SA-3 Goas, long rumored to have been supplied
by the Soviet Union to North Vietnam, however, the NSA (National Security Agency)
maintained that they were simply improved SA-2 Guidelines."

Any idea what system could that be?