Even if you've followed all the correct storage procedures, if, when you open the freezer bag, the meat looks odd or smells odd don't panic.
Bear in mind, that all meat that comes out of a vacuum pack will have a faint odor to it at first – it's normal. Because, as the meat ages it gives off a gas that contains sulfur, this gas gets trapped in the pack and is released when it's opened.
It's also quite normal for meat in a vacuum pack to be darker than normal. That's because it's been in an oxygen-free pack. When it's exposed to oxygen the color will return.
If you're concerned, do this:
Rinse the meat, gently, under cold water. Then leave it in a fridge, covered with a cloth for an hour or so. If, after that, the odor hasn't gone – and it hasn't regained its color – it's wise to bin it.