Excalibur
Geoffrey of Monmouth, in his "Historia Regum Britanniae" ("The History of the Kings of Britain", c. 1136), Latinised the name of Arthur's sword as "Caliburnus" (potentially influenced by the Medieval Latin spelling "calibs" of Classical Latin "chalybs", from Greek "chályps" ["χάλυψ"] "steel"). Most Celticists consider Geoffrey's "Caliburnus" to be derivative of a lost Old Welsh text in which "bwlch" (Old Welsh "bulc[h]") had not yet been lenited to "fwlch" (Middle Welsh "vwlch" or "uwlch"). In the late 15th/early 16th-century Middle Cornish play "Beunans Ke", Arthur's sword is called "Calesvol", which is etymologically an exact Middle Cornish cognate of the Welsh "Caledfwlch". It is unclear if the name was borrowed from the Welsh (if so, it must have been an early loan, for phonological reasons), or represents an early, pan-Brittonic traditional name for Arthur's sword.
Ebony Blade
Writer Paul Cornell finally revealed a full explanation within the Captain Britain series, having Dracula replace Dane Whitman's blade with a fake version at some point between "Avengers" (vol. 3) #37 and "New Excalibur" #10. This fake blade has a vampire fang within it, which has shown some signs of sentience as people have talked to the blade directly. This mystical sentience has acted in the same fashion as the original's curse; convincingly enough to fool Whitman. Even after Whitman first realized it was fake in "New Excalibur", it affected his memories so that he forgot, meaning he thought he held the real one by the time of "Captain Britain and MI-13".