Blues merles carry a merle and non-merle gene (Mm), compared to the non-merle dog (mm). Merle colouration is believed to be caused by the Silv gene, which is linked to pigmentation.
Merle dogs in any breed can suffer from slightly raised levels of deafness, compared to normal coloured breeds. A study on blue merle dachshunds, comparing them to non-merle dachshunds, found they had a nearly 9% increased risk of deafness. There is less clear evidence for the increased risk of blindness.
The thing is, no one is quite sure why the Silv gene causes this, though it seems linked with pale pigmentation around the ears and eyes.
Anyway, when you breed merle to merle (Mm) x (Mm), the mating has a 1 in 4, or 25% chance of a pup being a double merle. The litter will produce DMs (MM) along with normal merles and ordinary coloured dogs. (Mm, mm, MM, Mm).
While a double merle can have normal hearing and vision, many will have ear or eye deformities, to the point some have no eyes at all. These pups are often disposed of by the breeders as a consequence of the mating, unless they have really good merle markings, in which case they will be kept for breeding.
Now, as you say, why breed merle to merle, when the risks of a DM are so high? Well, when you have a DM (MM) you can breed them to a non-merle (mm) and guarantee a litter of all merles (Mm).
Does that make it ethical? Absolutely not, but some people want colour over health. Merle to merle breedings cannot be registered with the Kennel Club in the UK, which has largely stopped them occurring (I have yet to come across a UK double merle), though it would not prevent the breeding of merle to merle and a false registration (with a tri or sable parent replacing one merle) being submitted.
In short, it is not the sort of thing I would expect from a breeder who cared deeply about their dogs' health.