It can be blocked, yes, but this appears to be a fairly major jinx, the equivalent of a lot of capital equipment. Hogwarts is known to be anti-Apparation jinxed, as is Azkaban, but I don't recall any other places where it's mentioned. (Ministry of Magic, perhaps; implied by the workers there commuting in a fairly standard fashion instead of just Apparating from their homes.) It's not clear that it can be installed on an average wizard's home. Anyway, you'd think it would prevent inward, but not outward, Apparation by default; you don't want to be suddenly attacked but you might want to make a quick escape.
Dumbledore captures (in that fanfic by Rowling, that is, not the canon) some Death Eaters by preventing their Apparation, but he has to duel them first, so he might as well have cast AK.
All that aside, she's an experienced combat wizard with a few seconds to spare. If she can't Apparate, she still has the option og grabbing Harry, blowing a hole in the wall, and running. "Accio Broomstick", anyone? Or whatever flight spell Voldemort uses to go up the stairs without footsteps.
Come to think of it, why were they hiding in an apparently average home without special defenses, and relying on mere secrecy? Put them in Hogwarts, with its layers upon layers of magical fortifications. Draco states that Hogwarts is an impregnable fortress, and presumably Voldemort thinks so too since he doesn't attack Dumbledore there and end the war at a stroke.
This has come up before in the tvtropes discussion thread, but personally I operate on the assumption that apparating in a combat situation is simply beyond the skills of most wizards. It requires deliberation, which is probably hard to muster when someone is firing death beams at you, and if you tried it you'd probably get splinched. It's probably doable for really exceptional wizards, like Voldemort or Dumbledore, but not for your average auror, let alone the average joe wizard.
Highly tactically useful feats that are extremely difficult to perform would...
Edit: New thread posted here.
This is a new thread to discuss Eliezer Yudkowsky’s Harry Potter and the Methods of Rationality and anything related to it. This thread is intended for discussing chapter 86. The previous thread has long passed 500 comments.
There is now a site dedicated to the story at hpmor.com, which is now the place to go to find the authors notes and all sorts of other goodies. AdeleneDawner has kept an archive of Author’s Notes. (This goes up to the notes for chapter 76, and is now not updating. The authors notes from chapter 77 onwards are on hpmor.com.)
The first 5 discussion threads are on the main page under the harry_potter tag. Threads 6 and on (including this one) are in the discussion section using its separate tag system. Also: 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16.
As a reminder, it’s often useful to start your comment by indicating which chapter you are commenting on.
Spoiler Warning: this thread is full of spoilers. With few exceptions, spoilers for MOR and canon are fair game to post, without warning or rot13. More specifically: