This haki force field seems to be a form of advanced CoA similar to how Katakuri's future sight is a form of advanced CoO. Interesting to note that the admirals themselves already showed this advanced CoA when they collectively blocked Whitebeard's DF power.
The black blade might mean that certain conditions or haki usages might enable a permanent "haki enhancement" on weapons, as it did Ryuuma's and Mihawk's.