A blunt is, aside from the marijuana smoking method, a rather difficult skateboarding trick.
If a blunt had to be classified as a type of trick, it would be a stall, one that is performed on a lip or curb of some sort, and you simply stay balanced precariously in this position until you ollie out. Like most other stalls, if you slide along the edge of the object, it becomes a grind/slide, specifically a bluntslide or nose bluntslide.
So, what makes a blunt a blunt? A blunt is performed in a similar way to a board-stall, or a fakie-rock on vert. The difference is, instead of lifting your front wheels over the lip and resting on the middle part of the board, you keep going and let the rear wheels go over too. So, after this, you are balancing on just the rear truck and tail of the board. It's kind of like doing a manual on the top of the lip, only your tail is keeping you from going any further, since it is still hanging over the edge of the lip/curb. Another way to think of it is a vertical tailslide. After balancing for a little bit, you then pop the tail to push the rest of your board back away from the lip, and land. People used to grab their board and just pull it back away from the object, but that became passe in the early 90's. A basic variation of this is a noseblunt, where you ollie first, then rotate 180 degrees, and jam your board down into the same position described above. This is also sometimes called a nosepick.
The blunt was invented by Aaron Deeter. He started with the ollie blunt in Seattle in the late 80's. Oddly enough, he invented it on street, and then took it to vert. When it was first described, people thought it was impossible, and many wouldn't believe until they saw him do it.
Today, most normal blunts you'll see will be done on either vert or mini-ramps. However, the blunt has found a popular vein in street skating via the bluntslide, which is one of the most difficult grinds you will ever see performed. Skaters like Daewon Song have made a career of doing every combination possible of flips into and out of nose bluntslides, or else used a bluntslide down a huge rail to propel them to stardom.