There is no guarantee of a healthy puppy/dog, regardless of whether you buy from a reputable breeder, BYB, pet store, or rescue. Sadly, with living things no one person has total control - however - REPUTABLE breeders are usually involved in the breed for many years, and from that, can learn about longevity and health in various lines, ALONG with different testing, and aim to breed from, and for the healthiest dogs.
As well, when you are involved for many years - people often can keep 'track' so-to-speak of what you do, basically, you are there to answer for your mistakes, and should be taking responsibility for them as a breeder.
I personally have a hard time with bad ethics. While wins are nice, and breeding champions is of course wonderful, and the goal is improving the breed - I PERSONALLY put a higher priority on temperament along WITH conformation. I have seen many questionable temperaments in the past several years - in well-bred lines along with poorly bred ones.
I would recommend you talk to different breeders, meet their dogs - and understand sometimes meeting the stud is not possible - but no one should make excuses for bad temperament. (LOL Notice I dont say good temperament - Im always apologizing for my dogs trying to plant themselves in visitors laps!)
I also would say to be sure you buy from a breeder who sells on contracts - I personally sell on spay/neuter contracts with a mandatory return clause. This means that at ANY time in the dogs life it cannot be kept, it MUST come back to me. If a breeder sells a dog with no contracts regarding its well being - what makes you think they care about the safety of that dog, or anything else about that dog? A good breeder allows no pets to be bred, and doesnt try and place EVERY puppy in a show home either.
(I have found I prefer placing pups in pet homes than many show homes... so I rarely do.) I keep what I want to show, and place the others in homes where I know they will be loved unconditionally...
AND as for black Boxers... a personal pet peeve. They are incorrect, and technically, not desirable. NO ONE should be breeding FOR them. (I personally find reverse brindles incorrect when fawn is not showing through as described in the standard!) I cannot stand when profit breeders make up a new marketing scheme to con the general public. First it was white boxers, now black boxers.
Oh well... just my um..... 7 cents. LMAO