I believe that's the reference right there. Porn sites, along with most other sites, were usually not quite as spiffy as they are these days. Aside from some creative hot spots that really overdid it with the GIFs (geocities 4 life!) most sites were typically just browser defaults with perhaps some changes to background colors, maybe add a nifty tiled background image and such. Web design (not to mention development) was quite different back in the late nineties, early naughts, than what it's like now – the default blue link color was everywhere.

This whole issue is effectively a sarcastic response to that lack of design, and referring to the default blue as "the porn color" I read to mean "don't be cheap and stick with the defaults; don't be a porn site."

The joke doesn't age very well, but it was really funny back then.

It's "blueprint blue" - a reference to the cheap printing process used to make the posters for pornographic films, and also the reason they got the nickname "blue movies."

From the above Internet Archive link, looks like the color is #0033FF.