Heraldry is full of conventions and contradictions. One of those conventions is that the strap is the second livery colour of the armiger (in this instance, the Chief). Now the contradiction occurs when the second livery colour is blue. In that instance the usual choice would be black, taking the next colour in order of precedence (red, blue, black, green, purple). But, if so desired, you could stick with blue.

Now for all you heraldic hair-splitters: the "livery colours" are derived from the arms, with the first depicted colour in the wreath always being the first named metal (gold or silver, in that order). So even if the shield is blue charged with a gold thingiemabob, the liveries would be depicted on the wreath under the crest as gold and blue.

And that means that the Buckle and Strap would either be blue or black.