Interestingly enough, only the light one is in the Tartan registry.

Does anyone know why the darker one is not ?

I think the dark one looks OK, but it doesn't really stand out. There are seemingly dozens of dark blue/black/white tartans available, but the light Texas Bluebonnet is the only one I know of that has sky blue as the main color.

I would never stoop to ridicule someone's clan tartan because I don't personally find the colors attractive. As a native Texan, this is as close as we have to a clan tartan.