Thinking about this some more, you could have one of two problems with your MBP display. One can be fixed in software, the other can't. I'm not an expert in this area but did quite a bit of research in the past couple of years.
Possible problems
(1)
Black crush where very dark greys are "crushed" into absolute black. Common on LCD TVs using PVA technology. This can be fixed by changing the software calibration profile
(2)
Contrast ratio the difference in brightness between white and black, or putting it another way the blackest black the panel will show for a given brightness. This is a property of the pixels themselves and can't be fixed
The calibration page that aberrero linked to tests (1) above but not (2). Basically the calibration software maps software grey values to pixel voltages. Changing the calibration profile means that a given grey value will map to a different pixel voltage and will give a different grey on the screen. If the calibration is wrong all the low-level grey voltages will be too similar and will be crushed to pure black.
The fact that you can see only down to level 6, and I can see 6 or 1 depending on my calibration, is all about black crush, and is (mostly) not a fundamental property of the panel.
But if your problem is with the absolute black level (eg the background surrounding the numbered squares on aberrero's testcard, or the letterbox bars on DVD Player) you need to look at a different thing:- the contrast ratio.
I did a simple test just now, perhaps you could try it too...
- select a desktop background that's very black. I use the earth from space one, I think it is pure black (level 0 I think).
- looking at normal windows (eg Safari, Mail) set the brightness to a comfortable level. In my office which has "normal" light levels I am at about 75% brightness.
- hide all the windows so you can see the desktop. Don't use Expose since that leaves "edges" of the windows around the display.
- compare the black of the panel surround with the black of the panel itself. I can
just about see the difference in brightness between the two. If I turn the brightness down another 3 steps just about I lose the distinction. I have a little backlight bleed along the bottom so I am looking at the left & right sides of the display.
Is your black level significantly different from this (not very scientific) measurement.
If you want to measure your contrast ratio more accurately you could try this method using a digital camera....
http://www.lagom.nl/lcd-test/contrast-ratio.html. It's a bit fiddly and might take half an hour but at the end of it you will have a contrast ratio number which is reasonably accurate. I did this and got around about 600:1 - which IIRC is what the panel spec sheet says.
If you get any numbers or other results I'd be very interested to hear them