The article states they used a dark navy blue dye. See the sidebar "Five nearly-true ninja myths":
"Ninja clothing was made to be light and hard to see in the dark - but jet-black would cause the form to stand out in moonlight, so a dark navy blue dye was usually used"
Even that's probably not completely correct. If you want to blend in with what's behind you, you should look similar to what's behind you. The rules for this don't change much at night - in dim light or bright light, you still don't want to be a strikingly different shade to your surroundings!
"Ninja clothing was made to be light and hard to see in the dark - but jet-black would cause the form to stand out in moonlight, so a dark navy blue dye was usually used"