I tiled my tub surround last fall. A couple of weeks ago, I noticed one angle at the tub joint is darkening. A tile guy said the sealant might have failed and suggested I let it dry out for several days, then re-seal. Two weeks later, it's still dark (I haven't re-sealed yet). It seems to be embedded in the grout itself, the grout is solid and it won't scrub off. If the problem was mold or mildew it seems like the caulk would be darkening or damaged as well.
While I was working on this problem, I noticed some shadows in the middle of other grout lines. After some research, I think this is efflorescence, or the sealant is failing and it’s taking in dirt. Again, it doesn’t scrub clean.