Well, when I finally found the comments on the show, I see there was a more plausible explanation offered by Joshua Hudson, which said certain parts of ships were painted blue to indicate mourning. Still, the reference to coffin wood showed up on two official Marine documents I found, and I also found references to 2 different types of trees, sapele from W. Africa, and kapok found in Asia & South America, both of which were used for coffins, and both of which had bluish bark.
I do think the blue paint idea seems more directly related – but then, it leads to the question of how and why that tradition started. Oh dear, I thiink I need a nap.