EVASIVE ACTION BY LGT FLT CREW TO AVOID COLLISION WITH SAME DIRECTION SMT DESCENDING THROUGH THEIR ALT.

Narrative

CAPT WAS FLYING ACFT FROM HSV TO BNA (LGT X). AT 6 DME SOUTH AT SYI VOR AND 12000' MSL ASSIGNED; CAPT POINTED OUT FOR COCKPIT CREW AN SMT DESCENDING AT OUR 12 O'CLOCK POSITION LESS THAN 1/2 MILE DISTANT. WE WERE CRUISING AT 370 KNOTS (PDCS COMMANDED). SMT WAS FLYING AT 245 KT (125 KT RESULTANT CLOSING SPEED). WE WERE ON A COLLISION COURSE WITH THE SMT. CAPT DIVERTED COURSE TO LEFT OF SMT TO AVOID COLLISION. WE PASSED SMT LESS THAN 500' HORIZONTALLY; ZERO FEET VERTICALLY; TO HIS LEFT. SMT WAS IN OUR BLIND SPOT ABOVE 'WINDSHIELD EYEBROW' TIL CAPT NOTICED SUBJECT ACFT VISUALLY. WE ASSUME THAT SMT WAS UNAWARE OF OUR PRESENCE DUE TO OUR CLOSING FROM HIS SIX O'CLOCK LOW. SMT DESCENDED THROUGH OUR ASSIGNED ALT JUST BEFORE WE PASSED ON HIS LEFT. COPLT AND FLT ENGINEER STATED AFTER INCIDENT THAT MEMPHIS CENTER HAD PREVIOUSLY CLRED SMT Y FLT TO CROSS SYI VOR AT 8000'. WE LATER DETERMINED THAT THE SUBJECT SMT WAS SMT Y. SUPPLEMENTAL INFORMATION FROM ACN #87120: AS WE DESCENDED THROUGH 12000' MSL WE SAW LGT X GO AROUND OUR LEFT SIDE; SLIGHTLY ABOVE US; AND IN A WINGS LEVEL ALT. THE OTHER ACFT CAME UP ON FREQ AND COMPLAINED WE HAD DESCENDED THROUGH HIS ALT. THE CTLR ADMITTED THAT HE HAD MADE A MISTAKE.