The wiki you linked denotes event units on the comparison tables with a ★ next to the unit's name. This means you can find the number of event blacks by counting the number of ★s on the black unit page (most easily done with ctrl+f). A quick check shows there to be 13 event blacks, but that counts Dorania separately from Anya, and includes Liana (changed to a premium unit on Nutaku) and Daryun (exclusive to the All-Ages version of the game).
This is not correct. There are as of yet 3 item collection events we haven't gotten featuring platinum units, some of which have been quite recent. Specifically they are Arms Merchant and the Cursed Weapons, The Stolen Grimoire, and Wind God's Daughter.