IWC hasn't, as far as I know, disclosed the composition of the alloy, but one possibility is that it's partly niobium – there
link is a patent for a titanium
link alloy containing niobium from 2001, which says that heating the alloy to
link around 1500º Fahrenheit produces "a durable black surface layer consisting substantially of an oxide of niobium," but that's just (semi-informed) speculation.
link