Blacks not as good with Blu-ray

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I have a Samsung plasma B850, Sony BDP2000ES and Rogers box - all connected via HDMI. My picture settings are set the same way on my Bluray output as they are with my cable box.
The cable picture (I guess 1080i output) gives a great picture - dark blacks etc.
My Bluray picture (1080p) is visibly not as dark. Even with same settings (I won't go through them all). When I switch back and forth between the 2 inputs, the difference is very obvious.
Any reason why? Its very frustrating - i would have thought the bluray would give a darker, more saturated picture (it is sharper - but lighter)
Any responses would be greatly appreciated
Does the bluray player make a difference?