And Enma have ability to absorb haki within it. Then even Oden's dead it can store Oden's haki because it absorb so much before. Compare with shusui that after ryuma dead its has not ability to absorb haki within it self. Then ryuma haki go away when ryuma didn't try to infuse it.
DO I REALLY HAVE TO EXPLAIN YOU THIS BASIC SHIT ?
Compare shusui with enma is your ass headcanon from the begining. There wortless to answer your doubt headcanon with official. And is it show that how to make a permanent blackblade yet ?
There are wortless to use official for answer your doubt headcanon.
Even the officially cleary explaining it.
It officially confirm that Enma has Oden's haki within it.