A NGA ACFT ON A VISUAL APCH ON DOWNWIND TO FOLLOW A BEECH ACFT RPTED THE ACFT IN SIGHT; THEN HAS A NMAC WITH THE ACFT. RPTR BLAMES CTLR.

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DURING AN IFR FLT TO PORTLAND INTL ARPT; APCH CTL ASKED IF I WOULD ACCEPT A VISUAL APCH; I AGREED AND WAS VECTORED TO PDX AND HANDED OFF TO THE TWR. PROC WAS TO CROSS OVER THE CTR OF THE RWYS AT 2000 FT AND TURN L DOWNWIND FOR RWY 10L. I WAS CLRED TO LAND BY THE TWR. UPON TURNING DOWNWIND I OBSERVED A LARGE TWIN ENG ACFT ON WHAT APPEARED TO BE FINAL APCH TO RWY 10L. I WAITED UNTIL THIS ACFT WAS ABEAM MY L WING AND STARTED TO TURN BASE. TWR TOLD ME TO CONTINUE DOWNWIND WHICH I DID WHILE TRYING TO KEEP MY TFC IN SIGHT. AFTER A TIME I BECAME CONCERNED THAT THE TWR HAD FORGOTTEN ABOUT ME AND CALLED THEM. I WAS ALSO NERVOUS BECAUSE I COULD NOT SEE THE CAUSE FOR THE EXTENDED DOWNWIND AND HAD NOT BEEN ADVISED OF ANY TFC BY TWR INCLUDING THE ACFT I THOUGHT I WAS FOLLOWING. THE TWR ADVISED THAT I COULD FOLLOW THE TFC; A BEECH ACFT AT 11 O'CLOCK POS. I STATED THAT I HAD THE TFC AND ANNOUNCED BASE THEN STOPPED MY TURN BECAUSE I FELT THAT THE TWR WAS REFERRING TO A DIFFERENT ACFT THAN THE ONE I WAS FOLLOWING. I INCREASED MY SCAN AND INSTINCTIVELY LOOKED AHEAD AND TO THE R TO SEE THAT THE APCH LANE WAS CLR AND SPOTTED A DARK BLUE TWIN ABOVE AND SLIGHTLY TO MY L. I CORRECTED MY TRACK SO THAT I COULD AVOID A COLLISION WITH THE BEECH AND STILL MAINTAIN VISUAL CONTACT SO THAT I COULD MANEUVER SHOULD HE TURN; CLB; OR DIVE. HE PASSED OFF MY L SIDE; WINGS LEVEL AND SLIGHTLY NOSE HIGH. CONTRIBUTING FACTORS: THE TWR CLRED ME TO LAND WHILE; I BELIEVE; THERE WAS A TFC CONFLICT AND THEN CHANGED THEIR MIND AND EXTENDED MY DOWNWIND WITHOUT ADVISING ME OF MY TFC. THE ACFT INVOLVED WERE DARK BLUE AND EXTREMELY HARD TO SEE AGAINST BOTH GND CLUTTER AND THE DARK OVCST. THE BEECH MAY HAVE BEEN SIDESLIPPING ACROSS FROM THE ILS APCH TO RWY 10R. THE BASIC OF ALWAYS LOOKING R WHEN TURNING BASE ALLOWED ME TO SPOT THE BEECH IN TIME TO TAKE CORRECTIVE ACTION.