Greetings!
I have been involved in the Wilderlands design team and currently use a few parts of the draft in my home campaign. My knowledge about Blackmoor is limited, so take what I say with a grain of salt. (also, others will likely know more than I)
First of all: the two settings should be fairly compatible, and in the 70s, they were situated in the same world (although the Blackmoor map was on a scale four times less detailed than the Wilderlands). If you have the spare cash, you should probably buy both and put Blackmoor north/northeast of the "Valley of the Ancients" map.
What the two setting have in common:
-a very old school feel
-some remains of ancient
very high tech. This is more pronounced in Blackmoor, but still found now and then in the Wilderlands. In the new edition, you are given an option to use this particular aspect of the backstory, or consider it "high magic".
-the focus is on player characters. While there is a fair amount of powerful characters, they don't run the show, and a party of mid level (5 to 8th, even) could easily become power players in a region.
-there is a lot of wilderness. There are no "states" - rather, population and power is concentrated in autonomous city-states and hundreds of small, independent communities. Pretty adventurer friendly.
-power level isn't as high as the Realms or even Greyhawk. Level 10 is considered extremely high, and 20th level types are usually either extremely powerful movers and shakers or hidden from the normal world alltogether. Most town leaders are maybe 10th level if they are really powerful. In the new Wilderlands, there is an optional revised XP table that slows down progress to about halved speed.
Now, details particular to the Wilderlands, especially its current incarnation:
-The known world is mapped out on 18 poster-sized hexagonal maps with numbered hexes, broken down to the scale of 5 miles. This opens a mode of play that is often glossed over in today's games: wilderness exploration and even "striking out on your own" to see the wide world. The players also have their hex-maps, but these are incomplete, with only coastlines and well known features drawn in (so you may have a map where you can see the City State of the Invincible Overlord and the infamous Dearthwood, but not the really small and out of the way villages in the area - or, for that matter, distant lands). As they explore around, they have to map out their progress on their own maps. My players love this feature - in my campaign, getting t a certain dungeon was an accomplishment itself - they travelled from one map to another, and the whole process took half a year of game time and months IRL.
-The feel of the world isn't quite as medieval as other settings - more like the ancient Mediterranean with its morals, plus the usual D&D elements, mostly from Conan. Gods are above mortals but not by much (and often meddle), slavery is an accepted fact of life, evil clerics walk openly and so forth.
-There aren't too many commoners in there. Most inhabitants of the setting can fight as well as warriors and low level Fighters, Rogues, Wizards or Clerics are common.
-It is a bottom-up setting, concentrating on local details. It is very vague and easy to shape, especially at the global level. You could make it a dark fantasy setting, a realistic one (that one isn't too easy, granted), or as fantastic as you like. I made it a lot more conanesque and changed the feel considerably (with the removal of many demi-humans, etc.), which was really easy to do. The Wilderlands is a framework to change, play in and modify at your whim. This philosophy and freedom is central to Judges Guild and its owner, Bob Bledsaw.
For example, the "world book" will describe such things as villages, mysterious ruins, monster lairs, fortifications, small islands and say a few words about them with a short stat block or two. There is no metaplot, little details on "what is happening". That is your job.
-To see a few examples of how the end product may look like, I provide the following links, all of them officially released by Necromancer Games:
Official JG Site - has maps and news
Thread on Gazetteer
Boxed set teasers
More Teasers
Teaser on the City State of the World Emperor
City State of the Invincible Overlord Teaser
...
Hope this helps.
