Why is my MacBook Pro's screen going black for a second?

Clearly, something isn't right. I highly suggest you make an appointment at your nearest Apple Certified repair location or a Genius Bar.

If it's less than a second in duration - more a brief flicker than it actually all going black for a second - then I have a similar problem with my MacBook Pro (mid 2009 15" model). It's very sporadic, but it seems to be related to the computer being under heavy load (eg running 2 large virtual machines simultaneously) when it's running on the integrated graphics (in energy saver, using 'better battery life' instead of 'better performance'). I've never had it happen on the 'better performance' graphics setting, and I suspect it's something to do with the low-power graphics card.

I do keep meaning to take it to a Genius bar when I get a chance, because it's still under Applecare and it can be annoying (it tends to happen in a flurry of flickers for a minute or two, then nothing for hours on end). However, it's my main work machine and I'm not in a position where I could be without it for a few days, so.... :-)

This is an off chance, but I feel it's worth mentioning: this can happen if you have a hot-corner enabled and set to 'Sleep Display'. I do, and it'll occasionally happen that I'll accidentally move my mouse into the correct corner and my screen will go dark for a second before mouse movement wakes it up again.

Again, unlikely that this is your issue, but worth checking before you bring it to an Apple store. Just take a look in System Preferences (under displays or screen savers or something like that).