The best way to explain Tanzanite is to show you the cheaper end, a middle end (and this is what most people get and tend to call Cadburys) and then high end. This is just my opinion however and others may disagree.
First up, here's the lighter end of the scale which is the cheapest of tanzanite. This is what you'd call A grade. You'll see that the colour is very pale. Still attractive but you wouldn't want to be paying big bucks for this! I think this ring was around £70-80 a few years ago.
And another - high carat weight but not great Tanzanites:
This one is a little better but would be in the AA range and not above in my opinion.
Ok - so moving on to the mid range. This is what is in the majority of rings from jewellery channels. People calls this the Cadburys colour and it will often has quite a bit of purple as well as blue. This has good colour saturation but the depth is lacking. This isn't vivid but it's nice. This is probably better than an AA but only just inching into the AAA category (going by the Anchorcert gradings).
Not a great picture but here's another that's in the same category:
So now here are some fine examples of exceptionally good quality Tanzanite. Can you see how deep the colour is? Verging more on the blue end?
Dreadful photo as this was taken through the top of a gems box so it's taken through plastic but you get the idea!
A great great great Tanzanite is like looking at a piece of rich royal blue velvet (with a hint of purple). The only thing you have to watch is that sometimes Tanzanite gets too dark. If that's the case then it's not as valued as one that's vibrant. If you're buying a high end piece, make sure you see the ring (if it's a ring) on the hand and off the hand. Occasionally when you see a presenter put on a ring you'll see it goes black. You don't want that!
Hope that helps!