Filling a crack in hardwood floor (between house and extension)
Black caulk is what I used to fill the ~1/4" gaps in a timber floor hallway. The hallway consists of three, thirty foot long boards that are 13" wide and +2" deep. They have, what has become, a dark polyurethane finish, so I'd have to say it looks fabulous  (I had lived there for years and they began to split apart, so not having a gap that can cut you toes open is awesome in the first place). Yours will end up looking like a threshold, but there's nothing worse than light colored wood filler, IMO.

You can use a matching color if you want, but anything less than black with show dirt eventually anyway. It's kinda like grout lines, but for wood. But unlike grout (or the mixture of polyurethane and sawdust –from said boards– that we used to fill the gaps at first, it won't crack when the floor expands and contracts seasonally, or from further settling).

I do believe we'd urethaned over the whole thing at some point afterwards, and should it happen to flake off of the caulk (not that it has AFAIK), at least for a while it provided some level of protection from debris embedding themselves.



Specialty (grout) colored caulk is available for wood-to-tile transitions; look into that kind if you really want to match the color of your hardwood.