Vitamin D is especially important in climates where there is little sun. The majority of people who have blonde hair come form a place where there is very little sun for long stretches of the year. Theory makes sense when you consider the facts. It could help explain other things about evolution and vitamin D.
I have heard a similar study that white skin is actually part of a genetic mutation among people who drink milk. Their skin doesn't need to go into the sun to get their vitamin D because they get it from the milk and dairy product they use. Its only reasonable that blonde hair is part of this mutation.
Many studies are done and often, in the long run, they end up being looked up unfavourably. "Theory That Blonde Hair" doesn't sound very convincing which means I am inclined to place even less trust in it. As with all studies you need to read in to, or at least see the conclusions, to get a good idea. But at first glance the name is enough to make me think not to believe this study.
A scientific study recently put forth the theory that blonde hair is an evolutionary trait for Vitamin D synthesis. I would take this study with a hefty grain of salt: don't believe everything you read, especially when it comes to "scientific studies" that make incredibly dubious claims such as this one.