Blue outline of enemies

These are Empowered Demons.

Empowered demons are a game play mechanic which first appears in Doom Eternal, [...] Slightly stronger versions of the ordinary demons, they can appear in most of the game's levels as a dynamic element, and drop greater rewards when killed.

[...] The initial conceit of empowered demons is that they were demons who had killed other players during their own campaigns, and that the empowered demons would gradually grow stronger unless eliminated.

Note: the wiki does state that the outline should be yellow, however I have seen it to appear blue sometimes (having had played very recently - I don't know if this is an update, or a UI choice to affect colors).

Additional Note: the only thing that is blocking the outline is the Revenant in front of it. Outlines for Empowered demons appear through solid objects, but not other Demons (which can make it much easier to spot Spectres for example). You can also see the opaque section of the back leg obscured behind the door frame.