BHT407 PLT HAS NMAC DURING XSIT OF DEN CLASS B AIRSPACE.

Narrative

ESTABLISHED CONTACT WITH DEN TRACON 20 NM E OF DEN; ALT 6500 FT AGL; HEADING 241 DEGS MAGNETIC AND REQUESTED TRANSITION THROUGH B AIRSPACE WITH ARR INFO. RADAR CONTACT WAS CONFIRMED. FREQ CHANGED TO DEN TWR 12 NM FOR CLRNC INTO SURFACE B AIRSPACE AND INSTRUCTED TO DEVIATE ABOUT 20 DEGS R TO OVERFLY CTL TWR AND TWR WOULD CALL A TURN TO THE SW ON COURSE. AT ABOUT 10 NM E OF THE CTL TWR; I SPOTTED A WHITE CESSNA 310 EMERGING FROM BLIND SPOT OF THE R WINDOW SUPPORT AT SAME ALT; SBOUND; NO MORE THAN 400 FT DISTANCE ON A POSITIVE COLLISION COURSE. I PLACED THE ACFT IN A VERY STEEP DIVE; CALLED THE ACFT POS AND ESTIMATE WE PASSED UNDER BY 50-150 FT VERT WITH NO LATERAL SEPARATION. THE PAX ESTIMATED THE CESSNA PASSED 50 FT DIRECTLY OVERHEAD. THE ACFT WAS OBSERVED OUT THE L SIDE IN A GENTLE DSCNT AFTER WE COMPLETED A 300 FT ALTDEV AT 1/4 MI AT SAME ALT. IF EVASIVE ACTION HAD NOT BEEN INITIATED; THE RESULT WOULD HAVE BEEN A MIDAIR. I RPTED THE NEAR MISS AND EVASIVE ACTION TO TWR WHICH DID NOT HAVE THE ACFT ON RADAR. MOMENTS LATER; HE RPTED A PRIMARY TARGET ON THE BOUNDARY OF THE SURFACE B AIRSPACE. I WAS DIRECTED TO CALL THE TWR SUPVR TO PROVIDE THEM WITH ADDITIONAL INFO TO TRACK THE CESSNA. HE RPTED THE ACFT WAS 'TAGGED;' BUT PROVIDED ONLY MOMENTARY PRIMARY RADAR RETURNS WITH NO ALT READOUTS. THE RETURN WAS EVENTUALLY LOST IN TFC. I WAS ADVISED THAT ALL RETURNS WERE ON THE BORDER OF THE SURFACE B AIRSPACE 'AND THOSE PLTS KNOW WHERE THE LINE IS WHEN TRANSITIONING S.' ADDITIONALLY; HE INFORMED ME THE TWR OPERATOR MAY HAVE BEEN DISTRACTED BECAUSE HE WAS IN THE MIDDLE OF A RWY CHANGE. I DID ADVISE HIM THAT I WOULD BE FILING A RPT WITH MY COMPANY.