I just noticed some questions on the homepage show blue highlight. What is this new feature?
As explained in thei's answer, questions with your favorite tags are highlighted in blue. If you have no favorite tags, then it may still happen that the system guesses your "most frequented tags" and highlights questions accordingly. If you want to disable the latter, go to the prefs on your user page and add something like not-a-valid-tag to your favorite tags. This will prevent the system from guessing, and only questions with the tag you selected will be highlighted. There are no such questions – success!
And sorry, no, I don't know any better than this workaround.
These blue highlights are exceedingly distracting. (I've forgiven a lot for GUI changes here, but this is a bit much.) I've seen a few meta.SO threads, and no, I don't want to set "tag preferences" (unless there is a checkbox for permanently disabling this "functionality" in place); neither do I want to muck with Greasemonkey. Please consider a way to disable this, SE powers-that-be.
Your favourite tags show in blue.
As commented by Bill Dubuque: Or, apparently, your most frequented tag if you have not selected any favorites - see the "Frequented Tags" box above "Related Tags" on the questions page.
Maybe you refer to the functionality of your browser, where links that have been recently visited are in a different color?
I tried this: I chose a front-page headline in black, clicked to visit that question, then returned to the front page. Voila... now it was blue.