Will this cover my grey hairs completely or just blend them?

This is a toning shampoo, not a permanent dye, so it does not penetrate the cortex to lock in colour. On grey or white strands — which are highly porous — the pigment deposits readily and can look quite vivid, often making grey appear as a cool or neutral blonde highlight rather than disappearing entirely. If you have a high percentage of grey, the overall effect will be a lighter, more blended look rather than uniform coverage. On scattered greys the contrast with your base shade is softened, which many people find more natural than a solid block of colour. For full grey coverage you would need an oxidative permanent or demi-permanent colour. A strand test on a few grey hairs is the only way to see exactly how the shade translates on your hair.