[aallkkaa]

It's been years since I worked on a oUF layout, but in this case, I guess it would be as simple as this:

local oldTarget
local OnSpawnNamePlates = function(self, event, unit)
    if (UnitExists('target') and UnitIsUnit('target', unit)) then
        if oldTarget then
                oldTarget:SetBackdropColor(0, 0, 0, 1)
        end
        self:SetBackdropColor(1, 1, 1, 1)
        oldTarget = self
    else
        self:SetBackdropColor(0, 0, 0, 1)
    end
end

oUF:SpawnNamePlates("mnkNames", OnSpawnNamePlates, cvars)