NMAC BTWN A DO328; ON A R BASE AND A SINGLE ENG CESSNA XING UNDERNEATH ON DEP. BOTH ACFT WERE UNDER TWR CTL.

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WE CONTACTED BLI LCL APCH 8 NM S OF ARPT; AND WERE CLRED TO LAND ON RWY 16 VIA R-HAND TFC PATTERN. NO RADIO CALLS WERE MADE INDICATING POSSIBLE CONFLICTING TFC (EXCEPT FOR ONE ACFT INBOUND FROM THE W; THAT TFC; ON TCASII WAS NO FACTOR). APPROX 4 MINS LATER; WHILE ON A R BASE LEG; FO (PF) OBSERVED SMALL CESSNA (152; 172) PASS UNDER US FROM R TO L. FO HAD NO TIME TO REACT OR TAKE EVASIVE ACTION; OTHER THAN TO SLOW OUR RATE OF DSCNT. WE WERE IN A 500- 600 FPM DSCNT FROM 2000 FT MSL TFC PATTERN ALT; AND NEVER SAW THE CONFLICTING TFC; WHO APPARENTLY WAS DOING A R DOWNWIND DEP FROM BLI. WE RPTED THE NEAR MISS TO TWR; AND AN ACFT REPLIED 'THAT MIGHT HAVE BEEN US.' TWR SAID SOMETHING TO THE EFFECT THAT 'DID NOT KNOW HE WAS GOING TO DEPART TO THE N; THOUGHT HE WAS DEPARTING W.' ON THE GND; LCL SAID 'HE'S (THE CONFLICT ACFT) A LCL PLT; WE'LL TALK TO HIM WHEN HE GETS BACK.' I BELIEVE THIS INCIDENT WOULD NOT HAVE OCCURRED IF BLI TWR HAD ISSUED A TA TO EITHER/BOTH ACFT INVOLVED. ALSO; IT APPEARS THAT TWR WAS NOT AWARE OF THE LOCATION OF THE CESSNA AIRPLANE; EVEN THOUGH IT WAS WELL WITHIN THE ATA.