While marketing has some play in the trail ratings (I would assume, its only logical), trail designations are simply based on relation to other trails at the specific area. There is no common rating system. Since Nashoba was brought up, my home area, yes our blacks are relatively easy (probably the understatement of the year), however they are the hardest trails on the hill, hence their black diamond rating. On the other side, look at areas like Alta, where they do not have (or at least when I was there) any double black diamonds (they only went up to a single black diamond rating system). How many people have skied through avy warning gates to ski a single black diamond rated run (Alta has em)? Then, again, you have areas like Mont Orford (Sherbrook, PQ) which has triple black diamonds (3 of em), and they didn't really seem like anything other than a single to double black at Killington, but what got them their triple rating was the fact it was all natural snow.
So now, since they are area specific, now we get into the marketing aspect. Obviously a ski area wants it's trail percentages to match its expected clientels ability, so numbers may get skewed a bit here. But I also have to think about it from the instruction side, and how often we deal with clientel that is held back, not by ability, but by mentality. Having easier blacks are a great way to break the mental block many skiers get, when they are venturing into the realm of black diamond skiing, it allows them to build up their confidence and learn to utilize the skills they already know on new, more challanging terrain, without the mental break because of a trail rating. I think the most perfect example of this I've seen in recent years is Franz's Woods in Whistler, I think it is probably the easiest black diamond there, but compared to some other black diamonds, Franz's looks and skis like a mild blue (and was no harder to ski than a balck diamond at Nashoba).
Now another reason that I just thought of, might be based on expenses (business related trail ratings I guess I'd call it). If a ski area doesn't want to constantly maintain, blow snow (if man-made is available), or just can't really let a trail be used to much (maybe environmentally based here) they will rate it higher to disuade skiers from skiing it, since after all the most commonly skied trails are typically greens and blues.
So what all trail ratings has everyone seen:
Green Circle - beginer terrain, cruiser, mountain road
Blue Square - intermediate terrain, open cruiser, small bumps, light pitch
Blue Diamond - high intermediate terrain, easy black pitch but groomed
Blue Square/Black Diamond - see Blue Diamond
Red Triangle I believe (europe mainly I thought) - European version of a blue/black I think
Black Diamond - expert terrain, often hardest at the area, steep, narrow, bumps, natural snow, etc - possible avy danger
Double Black Diamond - expert terrain, often hardest at the area, steep, narrow, bumps, natural snow, obstacles, etc, a little harder than a single black at the area - possible avy danger
Triple Black Diamond - expert terrain, hardest at the area, steep, narrow, bumps, natural snow, obstacles, etc, a little harder than a double bllck at the area - possible avy danger
Extreme (I've seen as a red trianlge with a white center and an !) - FUN!!! (but typically somewhat dangerous and "no fall" zones) - avy danger