I have fed Harlan-Teklad lab blocks in the past and have had the exact same results in appearance as feeding a good homemade mix high in protein with my "secret ingredient".

Which is much healthier for the mice than lab blocks anyway.
I did purchase, last year, several breeding mice from a breeder who lives in states where GM crops were giving farmers problems with their pigs and cows reproducing. This mouse breeder had been feeding a lab block high in corn and soy content. I bought mice from a couple other breeders at the same time that were from different states and fed different diets, after several months, all the mice from the other breeders were reproducing just fine. The ones from the other breeder either did not get pregnant or had extremely small, weak litters. Several males purchased also never reproduced. Only one doe actually produced a healthy litter and then mysteriously died before she could be bred a second time.
Of the remaining does, one of them, now almost 9 months old, just had its first litter (despite being housed with bucks her whole life) and only produced ONE baby. Another doe had 4 babies, and all but one were culled because they wouldn't gain weight. One doe got massively pregnant and remained pregnant for 2 1/2 months before I culled her.
I had no problems with any other mice purchased at the same time and all the does that did eventually get pregnant from the one breeder had been on a soy/corn FREE diet for almost 3 months before they conceived.
I can't say for sure that it was the corn/soy that did this, and several breeders have disagreed with me, and, of course, as usual, they tried to say it was my fault and something I had done wrong (despite the fact that I was having fat, healthy, successful litters with mice acquired from 3 other breeders). And I haven't had any problems with breeding/smaller litters with any other mice purchased since then from other breeders or breeding from stock I've bred.
I believe this had to do with the lab blocks/GMOs in the diet of these mice. I think the location you are in effects how at risk your lab blocks may be. Anyone can feed anything they like, its their opinion, but I choose a soy/corn free diet for my mice and achieve the same results that I had with lab blocks.
On a side note, another breeder in the same area was feeding a strictly lab block diet, and all of her mice suddenly stopped reproducing. Several factors were believed to be the cause, but none of them really made any sense other than the lab blocks. Again, this can't be proven, but thats just my opinion.
Lastly, as I said anyone can feed anything they like, but I would like to put a scenario out there: If you had a sick breeding doe (let's say it was super-nice, a perfect specimen) and you had two options for treatment: Tetracycline -which has been known to cause sterility in does or Baytril - which will only effect an already pregnant doe's offspring. Which would you choose to treat? Most would choose the Baytril because the risk of sterility is almost unheard of when compared to Tetracylcine (where the risk is very high). So why would you choose to use lab blocks which could potentially cause reproduction problems (and it may take 3 generations for these problems to show up, but by then, it could be too late!) when you could make your own mix and have mice that look just as good with little to no risk of reproduction issues?