A C208B PLT ON AN IFR FLT PLAN CLRED VIA THE ARVIN ARR TO BFL EXPERIENCED AN NMAC.

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FLT FROM ONT TO BFL. A FEW MINS AFTER DEP FROM ONT; I REQUESTED VFR ON TOP 10500 FT AND DIRECT TO PMD VOR; THEN ARVIN 1 ARR TO BFL. AFTER SOCAL (125.5) HANDED ME OVER TO JOSHUA APCH (124.55); I REQUESTED DIRECT TO ARVIN INTXN BUT THAT REQUEST WAS DENIED BY THE JOSHUA APCH BECAUSE OF TFC. MY DIRECT FLT HDG FROM ONT TO PMD VOR WAS APPROX 315-320 DEGS. AFTER PASSED PMD VOR; I RESET THE COURSE 298 DEGS TO JOIN ARVIN 1 ARR; THEN I NOTICED UNKNOWN ACFT WAS APCHING SUDDENLY MY 2 O'CLOCK POS AND STARTED EVASIVE R TURN WITHIN A COUPLE OF SECONDS. I WAS USING AUTOPLT COUPLED WITH ALT AND NAV (GPS) MODE AND I DIDN'T HAVE A TIME TO REACT. C208B (CARAVAN) HAVE VERY GOOD VISIBILITY FROM 10 O'CLOCK - 2 O'CLOCK POS BUT 3 O'CLOCK AND 9 O'CLOCK POS HAVE VERY LIMITED VISIBILITY. OUR 4 O'CLOCK - 8 O'CLOCK POS PLT'S BLIND SPOT. I THINK THIS UNKNOWN ACFT APCHED FROM ABOUT 4 O'CLOCK POS AND ALMOST CUT IN FRONT OF ME WITH COLLISION COURSE. AFTER HANDED OVER TO JOSHUA APCH FROM SOCAL; THE APCH CTLR POINTED OUT TFC; THEN THIS CTLR GOT BUSY WITH 1 PLT; GIVING HIM A HOLDING INSTRUCTION; WX AND SO ON. THIS CTLR WAS KEPT BUSY ALMOST TILL HANDING ME OVER TO BFL APCH. SOUNDS LIKE THIS CTLR DIDN'T NOTICE WHAT HAPPENED OVER THE PMD VOR. VFR ON TOP CONDITION; PLTS ARE RESPONSIBLE TO SEE AND AVOID THE TFC AND LIMITED TA FROM ATC. MY CASE; I WAS NOT ABLE TO SEE AND DIDN'T HAVE ANY WARNING FROM JOSHUA APCH. I THOUGHT ATC RADAR HAD TFC COLLISION WARNING.