Edit: More explicit explanation of why this answer is constructed the way it is.
There are actually multiple questions hidden in what you're asking:
- Why is the voting results map so overwhelmingly uniform?
- Why is the resulting colour blue, ie. why is it uniform in favour of Dems and not Reps?
Most existing answers concentrate on #2, but answering #2 without addressing #1 is going to result in a potentially very misleading picture of things. Those are distinct, albeit heavily interrelated questions, and both have the same underlying cause: Because the US voting system doesn't allow a third party ever to rise to power.
Since it doesn't allow any meaningful political spectrum to arise, people have to choose the lesser of two evils. While Democrats are commonly seen as being liberal or "the left" (which are the policies much more likely to be favoured by younger people, people from urban areas, people with higher education, and people with more exposure to different people and cultures) in the US, they're actually shockingly far to the right and conservative by almost anyone else's standards, and their platform often boils down quite literally to "at least we're not Republicans". If the US didn't use the very broken first-past-the-post voting system, you wouldn't see a sea of blue, and instead you'd see a rainbow of options, mostly skewing to the left of the national popular vote.
So what you're seeing is an artefact of the fact that:
- The US political system uses a known-bad voting method which invariably leads to a two-party deadlock, and by extension
- Most voters will find themselves in a situation where they're choosing the lesser evil rather than choosing someone who truly represents them, and by extension
- When one of the evils becomes particularly abhorrent to a particular section of the population (as is the case with Trump and young, urban, cosmopolitan population of a college town), they will almost invariably choose the other one
Whilst it might seem very odd and unusual to see ~90% of votes for one candidate in a particular place, it's really no more unusual than the fact US elections almost always end up with close to perfect 50% split, which is probably what you're used to and consider normal, and is just another facet of the same system that causes both.
If you're interested in exploring the topic of first-past-the-post and its problems, other voting systems, and how voting systems really shape almost everything about the political system, CGP Grey has an excellent series of videos on that.