Not only do they have most of their genes in common, but it is important to remember that most phenotypic traits are a result of work of multiple genes in different time of the development of the individual. A lot of genes do not produce things characterized easily, like the pigment color and deposition in skin and hair, but they regulate the expression of other genes or activity of individual cellular processes.
As we do not know, what is the function of the genes which are different between wolf and dog (I am actually not sure in what way the genetic difference between the two is specified - but I doubt it is on the level of genes), we really do only select for some physical/behavioral traits which are actually a result of both genotype and environment.
So, really the wolf-blood percentage has absolutely no relevance once you do any selection, it is only relevant in completely random combination of the parents and only works on average in statistically relevant size of the groups you try to characterize. Theoretically, even absurd things are possible with selection. For example, if the only gene that was different between the dog and wolf was eye color, and it was just dominant single-gene trait, then you can take a wolf and mix every generation to dog only, as long as you pick up only the yellow-eyed puppies, you will have a 50% wolf (according to genes) for ever, even thought the "wolfblood content" decreased to almost 0. Of course that's absurd, but statistically, mixing 50% with 100% gives 75% in average in population, which would be true, but the population will consist of (again statistically) 50% of 50%wolf/dog, and 50% of pure 100% dog.
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