It's a wooden bokken coated with blackened haki.
“It’s a wooden sword turned black”
“It’s a sheathe”
“It’s a sheathe that he turned black with Haki”
“It’s a sheathe painted black that he then turned black with Haki”
“It’s a painted sheathe, turned black with Haki, concealing a wooden blade that he also turned black with Haki despite using neither sword nor sheathe in combat”