First hole overblow
First hole overblow is very difficult to play and I think it doesn't sound well, even blowed by the better players.
Yesterday I listened a Carlos del Junco song and I think he hitted that note at a song moment, He is a great player but that note was terrible.
My opinion is that overblowers use overblows mainly as passing notes, They don't use to stay very much there, just like people like PTGazell using half valved harmonicas, and I thought: Is a good idea to valve the second hole blow reed in order to obtain the flat third in first position?
The better overblowers can actually sustain the overblow and can even bend it up in pitch. This takes a harp that is well set up by a really good tech (or, with a lot of practice, you can learn to do it yourself).
That said, yes, the Hole 1 OB is the hardest to achieve. The biggest problem with the 1OB is that you have to get the blow reed to stop sounding, and the blow reed in Hole 1 has to be gapped high enough so that it can get moving in response to your breath during normal playing. This makes it harder to get the reed to stop sounding when you play an overblow. So you often get both the blow reed and the overblowing draw reed sounding at the same time.
With enough response time, though, you can achieve the 1OB and sustain it. For instance, I just tried the following exercise on a few different harps:
Exercise: Play without any pause, Blow 1, then Draw 1, then Overblow 1, and sustain it for several seconds:
Chris Michalek custom Marine Band in G: Couldn't do it
Filisko custom Marine Band in A: Could do it after a few tries
Stock Seydel 1847 in Bb: Worked OK
Marine Band Deluxe in D with minimal self-tweaking: Did it easily.
Note how it got easier as the key of the harp got higher: The lighter weight reeds were easier to get in motion at the desired pitch.
(I wish there were a way to attach audio files to posts so that you could hear this.)
One trick you can use is to squirt air from your mouth to get a brief overblow in Hole 1. You do this by
1) Using your tongue in the K-Spot position to completely close off the air passage between your mouth and the rest of your breathing air column;
2) squirting air out of your mouth by quickly raising your jaw to expel air into the harmonica.
This technique cuts the resonant frequency of your mouth in half (creating a closed tube instead of an open one), but can't be sustained, as opening the passage to your air column destroys the resonant chamber.
However, if you prefer to valve Hole 2, that's a perfectly valid choice to achieve that elusive note in the chromatic scale.