There are - or at least used to be - a lot of myths about white boxers and health problems. But there is no evidence to prove that to be the case.
The exception is deafness (not blindness, so forget about that one) - and there is a plenty of evidence to show it is the case. We even know the specific gene responsible - and it just happens to be the same one responsible for white markings/coat colour. White on a boxer is not a colour, per se, it is lack of pigmentation - exactly as it is in Dalmations. That lack of pigmentation (or lack of melanocytes) causes suppressed blood supply to the cochlea, resulting in deafness. End result is that around 20% of white boxers are deaf in both ears (versus about 2% for coloured boxers). Most likely a similar number are deaf in only one ear.
If you've found a breeder and vet who say otherwise, I'm afraid they are wrong and could use a little education on basic boxer genetics. You could point them to these threads/sites for a crash course:
http://www.americanboxerclub.org/white-deafness.html
http://www.steynmere.com/DALM_DEAFNESS.html
http://www.boxerworld.com/forums/showthread.php?t=34760
Of course, if you've lumped questions about deafness in with blindness and other health defects supposedly attributable to whites but for which there is no evidence, it is perhaps forgivable that they've refuted that.
What is worth noting here is that if a white boxer is going to be deaf, it will be so from an early age (around 3-6 weeks,
not actually from birth). So if you're looking at purchasing one at the correct age of 8 weeks, it should be perfectly evident whether or not he can hear. Take someone with you when you go to see him and have them make a loud noise in another room (a whistle is good, but not something that causes vibration like stamping). If he can hear, he'll look in that direction. If you particularly wish to know whether or not he is deaf in one ear, you'd have to take him for BAER testing.
If he is not deaf at 8 weeks, he will not become deaf later for any reason related to being white. Deaf boxers also make perfectly good pets and can be very well trained using hand signals. LOL - since dogs don't actually speak or understand English, there's really no difference to the pup if you teach him to respond to a voice command for 'sit' or if it's a hand signal instead.