I used to read a lot of Jakob Nielsen's writing, on his website and books. I don't read his work so much now because I find what he says tends to be too conservative. He tends to be too strict with guidelines. For years he insisted that all unvisited hyperlinks should be blue and all visited ones should be purple.
One of the tricks with usability I've found is knowing which rules to stick to, which ones to bend and which ones to break.
Back on the first version of my site some years ago, I tested blue links versus my design's default (orange, if I recall correctly). Blue won by a mile.
Your mileage may vary. I've also had folks cite usability experts who suggest that changing navigation options is a Very Bad Thing. I do it when folks go to the purchasing page. It is a Very Bad Thing That Pays My Rent And Then Some.
I'm not in the 'all links should be a hideous default RGB blue' camp, but we may as well note that the readers of this site are not similar to the target audience of the average website in terms of technological sophistication.
I, for one, would rather that the main article links and comment links were blue/purple. The visuals on here frequently distract me or make certain tasks harder than they should be. "Managing" is not the same as a good experience.
I agree, it's a sensible guideline, but he would be critical of websites that don't follow this guideline.
It's good to be consistent. But lots of sites now have an colour scheme that makes it clear which links are visited and which ones are unvisted. Generally the visited ones are a duller / subdued.
One of the tricks with usability I've found is knowing which rules to stick to, which ones to bend and which ones to break.