In some publications it is stated that an overblow is played in much the same way as a blow bend. "What kind of blow bend?" would be the first thing I would like to ask. Is that a tongue blocked blow bend or a puckered blow bend? If it is a puckered blow bend then is it a tongue tip bend or a H-spot bend?
A number of publications describe a technique which uses the tip of the tongue to force the overblow to sound. This, in my opinion, is a very bad way to learn to overblow. It requires a lot of tension in a number of muscles that are not used in playing other notes like normal draw and blow notes or bend notes (unless of course you use the tip of the tongue to play bend notes, which I would also advise against). This tension will make smooth transitions from one note to the next nearly impossible.
The tip of the tongue and the muscles in the lips and cheeks are not very stable muscles, especially at the force needed. The tip of the tongue will tire quickly and the control needed to play a longer note at the right pitch will be lost.
The method I advocate is one where the overblows (and the draw and blow bends) are played using the H spot. The H spot is the part of the tongue which stops the airflow in pronouncing a rapid series of "hey" sounds.
I know this is a very inaccurate description, but the point is that the action takes place far back on the tongue at the start of the throat and not at the tip of the tongue.