Hi @peg_T
Let’s start with the most misunderstood thing about overblows (and overdraws as well): you do not blow harder (or draw harder) to get the resulting bend !! Doing so risks damaging your harp and also developing bad playing habits. 
To explain what an overblow is, we need to recognize that the terminology (“overblow”) is very unfortunate, because an overblow is simply a form of blow bending, but does not involve more force when blowing, as its unfortunate name might suggest.
Most serious harp players know that it is possible do a blow bend for the holes 8, 9 and 10 (also 7, but that blow bend is not needed, because it gives the same note as -7). It is not as easy for most players to do a blow bend when compared to a draw bend – but it is relatively easy to learn compared to an overblow bend !!
We also need to rehash some details about the reeds and notes found on a Richter tuned blues harp (the “standard” type of harp sold almost everywhere). If you pay attention and play each hole carefully as a blow note, and then as a draw note, you should be able to hear that for the holes 1-6 (inclusive) the pitch of a hole’s blow note is lower than the pitch of its draw note. This is very important in what follows later below.
However, for the holes 7-10 (inclusive) the pitch of the hole’s blow note is higher than the pitch of its draw note !!
Without going into the mechanical physics of bending, I now will simply tell you that there are two types of bends you can play when blowing: one type is only possible for the holes 7-10 and is commonly called the blow bend. The blow bend on one of these holes results (when done correctly) in a note that has its pitch a half-step lower than the unbent blow note for that hole. The usual tab used for these blow bends is, for example, +8’ or +9’ (for hole 10 two blow bends are possible: +10’ and +10", the latter of which is a half-step lower in pitch than the note +10’).
The other type of bends you can learn to play when blowing (although they are quite difficult and often require adjusting the harp’s reeds to play them at all) are only possible for holes 1-6 and are commonly called an overblow (some call it an overblow bend). The result (when played correctly) is a note whose pitch is a full-step higher than the normal blow note in that hole (it is also a half-step higher than the pitch of the normal draw note in that hole). The usual tab for an overblow is, for example, +1o or +4o etc.
Really skilled players (Howard Levy is the best example) can bend these overblow notes even higher than I just indicated. Most players can eventually learn to overblow, but the result is very often not accurate (pitch is too high or not high enough) or very unstable or only sometimes playable (which makes playing them “on stage” a hit-or-miss affair) – and for this reason many professionals never or almost never use them.
I will not delve into overdraws (possible for holes 7-10), as they are even more difficult and almost always require setting up the reeds in a specific way in order to play them.
Artists who regularly use overblows, blow bends and overdraws: Howard Levy and Jason Ricci are good ones for you because they are not only skilled but they use them frequently (especially Howard Levy).
Regards,
– Slim 