Learn English – word for love of nightmares/dark/macabre things

Omniphile: omni = every; -phile = love. Xeno- = foreign things, so xenophile means lover of foreign things, not lover of everything.

Wiktionary gives billet-doux, which is technically a borrowed word, but is used in the English language in the same manner as any borrowed term, such as crime passionnel.

The only alternative I can think of is the conditional term "Valentine" which can mean "An expression of affection, especially romantic affection, usually in the form of greeting card, gift, or message given to a person the object of affection, especially on February 14th.". While it is conditional, it is the only single word term for a love letter that I could find.

Otherwise it appears that there is no single word term.

As a last note, I would like to point out that the noun "rejection" is not commonly used to refer to a rejection letter; if you have heard it used as such, it was probably being used as a colloquialism or in a context where its meaning was obvious. For example a situation in which a person holding a letter states "It's a rejection". Even in that context, its usage would be uncommon, people almost always say "rejection letter" in full.