Will it fully cover my gray hair, or just blend it in?

The product description says it "helps gray strands blend in more evenly" so the overall color looks richer and more polished. That wording is deliberate: on gray or very light hair, the deposited pigments show up more strongly because the base is pale and porous, but the result is still a translucent veil of chestnut tone, not the opaque, uniform coverage you'd get from permanent color. If you have a high percentage of gray (say, over 30–40%), you'll likely see a highlighted or salt-and-pepper effect rather than a single solid shade. For first grays or scattered strands, the blending effect can be quite natural. If full coverage is your goal, permanent dye or a professional service is the more reliable route. Also note that porous gray hair may grab the tint more intensely, so a strand test (as suggested in the leaflet) is wise before applying to the whole head.