To achieve a more natural look when colouring mens beard hairs, use a product like Blackbeard for Men
You should not be able to tell you are using Blackbeard for Men. In fact, one of the hallmarks of the product is that it allows you to create a more irregular colouration, as opposed to a dye which creates a uniform, artificial look.
Here are a few tips:
1) Use a facial tissue or a beard brush to blend the colour. You can lighten the product as needed to ensure a natural look. Uniform colouration is a giveaway, so vary the intensity in places.
2) Leave some grey if you like for a natural look.
3) If your hair has a bit of sheen to it, use a tiny amount of beard oil and a beard brush to give your beard a bit of lustre.
4) If the colour just isn't blending well with your facial hair, try a shade lighter or darker. Remember, you can always vary the intensity with a tissue or beard brush.