Here’s a little sample of some overblows and overdraws in action. The guy in the video playing harmonica is somebody who I’ve known for almost 2 years and is my harmonica teacher. His name is Todd Parrott and he’s a very well-known overbending harmonica player out of Charlotte, North Carolina. In this clip, he’s using every achievable overbend on the diatonic harmonica.
Now to answer those questions: Overbends can use either of the two main embouchres, lip pursing and tongue blocking. I can’t say about overbending with U-blocking because I don’t play with it. Neither do I tongue block overblows. In terms of tone quality, it ultimately depends on the gapping on the harp. Often, an OOTB harp can barely overblow while a custom one can overblow immediately. That being said, you can get overblows easily with a little practice. It just ultimately depends on what you want to achieve on the harp. If you want to play more chromatically or have more expression in the upper register, then learn to overblow