>"purple" seems like an odd color for a youthful blush. It would
>be more fitting as a description of rage, drunkenness or high
>blood pressure. Why not "rosy"?

On one hand, "rosy" is indeed the appropriate word for "a youthful blush," as in:

Love's not Time's Fool, though rosy lips and cheeks
Within his bending cycle's compass come
-- Shakespeare, "Let me not to the marriage of true minds..."

On the other hand, if the blood is supposed to signify something morally questionable, then the literary convention is to identify the color of the blood as "purple," as in:

The purple testament of bleeding war.
-- Shakespeare, _Richard II_, act 3, scene 3

Cruel and sudden, hast thou since
Purpled thy nail, in blood of innocence?
-- John Donne, "The Flea"

The decision to be made by someone translating Ovid's "ruborem" is whether to stress the surface of Narcissus's beauty, which is rosy, or the moral taint behind it, which is purple. If Tom is right about Eliot alluding to Ovid, then "carbuncular" is Eliot's purple version of "ruborem."