Can you watch Blu-Ray?

Sorry No.

Mac does not support Blu-Ray yet.

Apple does not support blu-ray.

But you do have other options.

1. Enable "Target Display Mode" on your late 2009 iMac and connect a blu-ray player using HMDI to mini-DP.

2. Install Windows on your iMac, either via Bootcamp or Parallels/Fusion, attach blu-ray dive via USB, and play blu-ray using Windows software.

3. There is now ablu-ray playerfor MacOSX that you can try with your external blu-ray drive.

...to watch Blu-ray discs on either?

Assuming you mean to watch commercial movies you purchased on Blu-ray disks, first you need to get a Blu-ray drive, as no Mac has a built-in Blu-ray drive. Second, you need to get some software to play the movie. For example, Macgo's Blu-ray Player. Initial reports said that the software was a little buggy so proceed at your own risk.

Apple can't support Blu-ray Player is true. But Mac could support Blu-ray player, because My Mac os could playback Blu-ray Player with" Macgo Mac Blu-ray Player."