What you've got is an attribute called warning, which the Valkyrie Quest Artifact, the Orb of Fate, provides as an extrinsic while carried.
Warning lets you know about dangerous foes that you may face, when you cannot see them directly (such as through walls, or blinded in your case). The color of the warning is related to the relative strength - dark red means it's a low danger, while bright pink means it's among the most dangerous of the denizens of the labyrinth. The more basic creatures that you met from the start are too weak to even be detected by warning.
In the ASCII version, these are actually shown as numbers, 1-5, which correspond to the actual threat level. However, according to Nethackwiki, the graphical interface uses question marks instead, making color the only way to distinguish the threat levels. And, on a second look, apparently the color of the question marks is different than how it is in the ASCII version, so apparently orange is the weak color instead of dark red.