Here's a post of mine from the Harp-L (SPAH) list, covering U-blocking and how to do it.
I've often read that U-blocking requires a genetic capability to curl the tip of the tongue into a tight little roll, and that only 50% (or some other irrelevant statistical value made up on the spur of the moment) of the general population can do it. That's not what I use - the TIP of the tongue is NEVER curled around a hole. (Maybe I have a different genetic "mutation" of the tongue from other U-blockers!) It's also stated (by a lot of the "experts") that U-blocking cannot be "taught"; I disagree (obviously).
Here's a little experiment that you can do to try the U-blocking embouchure.
Open your mouth wide (like you would do for a dentist who is about to insert his hand and a dental drill into your mouth). Keeping the mouth open, relax your tongue as much as possible, and at the same time, stick out your tongue just a little so that the bottom of your tongue (NOT the tip of the tongue) is at least touching your bottom lip. Lay your index finger down the middle of your tongue (without inserting it too far and invoking the gag reflex) and press down slightly (as if using a tongue depresser). Now open the back of your throat while taking a deep breath, which will cause your tongue to move lower down into the lower jaw. You should feel a slight suction pressure on your finger. Pop your finger loose from the suction (leaving it in your mouth, lightly touching your tongue). You should feel a slight "groove" running down the middle of your tongue. That "groove" forms the "U" or channel for U-blocking.
Now remove your finger from the mouth, and stick a harp in your mouth as deep as you can get it. The harp sits on top of the tongue, not at the tip of the tongue. Your tongue will naturally form a "U"-shaped channel around a single hole (with a little practice). A trick that might help a little initially is to tilt the front of the harp (the part outside your mouth) upward a little to start with - no more than 30 degrees. I learned that trick from David Barrett, who uses the curve of the lower lip to form a seal around a single hole. It really doesn't matter where the "curve" is located that surrounds a single hole - mouth, lip, tongue; the important thing is to place the harp so that the "curve" naturally wraps around a single hole on the harp.
The nice thing with this embouchure is that you can get the harp really deep into the mouth, which helps tone. I've found it's really simple to learn how to bend using it, by simply moving the base (back) of the tongue toward the back of the throat while articulating from "EEEEEEE" to "OOOOOO". It also allows for tongue slaps, pulls and chords, although not necessarily the same chord position (root, first inversion, second inversion) as when tongue blocking. Octaves (double stops) also seem to be doable without much trouble, although it would be technically correct to call that technique "tongue blocking". If the tongue is wrapped around a hole or two (not really wrapped around it, but you get the idea), it's relatively simple to just push the tongue forward to close the "U"-channel without filling the lips and Voila! you have the octave.
I tried teaching this embouchure as an experiment at our local harmonica club. We have one member who has been learning harp playing for about 2-3 months, and one new member who visited for the first time. Neither of them could play a single hole cleanly. I spent some time with them, describing in detail the 3 major forms of embouchure (pucker, tongue block, U-block). When I discussed U-block, I described the supposedly genetic requirement to curl the end of the tongue into a tight little tube to wrap around a single hole note. Guess what?!? Neither of them could curl their tongue. However, I used the same description given above, demonstrating how to do the "tongue depression" of the tongue with my mouth open wide. I asked them to try it, and both of them were playing clear single notes in less than 1 minute. Neither of them were able to blow clear single notes using either of the other two embouchures.
"One or two swallows do not a summer make", but I think it at least shows that just about ANY embouchure can be learned, if taught properly. It will be interesting to see if either (or both) of them are playing single notes using U-block or one of the other embouchures at next month's meeting.Try it - you might like it!