A variety of compounds are used to make black tattoo ink. Generally, synthetic black ink is made of iron oxides and carbon, while natural black ink is made from materials such as magnetite crystals, wustite and logwood (see Reference 1). With precise skill and exacting application of the correct type of black ink, a trained tattoo artist can make black tattoo ink darker (see Reference 2).
Select the appropriate ink for the desired black content of the tattoo. Some inks, such as ones that are made from large particulate pigments, won't penetrate the skin deeply, but will appear darker near to the surface. Inks such as Pelikan India Ink and Talens India Ink work well with dark black tribal tattoos.
Chose a needle that works best with the type of ink that you select. For example, inks that have larger particulate pigments require a medium-taper needle.
Keep the needle at a low angle and the stroke longer. With a low angle, you will tattoo a much darker series of layers, rather than a shallow angle perpendicular to the skin. Also, keep in mind that you may need to spend more time with lower pigment inks, while also using a faster machine. This is because with lower pigment ink, you are putting less pigment into the skin with each stroke.
Set up many small ink caps for dark black tattoos, rather than one large ink cap, especially if the tattoo is large. When you continuously dip into the same ink cap, blood particulates can contaminate the ink supply, both diluting it and adding coagulants, which will cause the ink to form blocks that do not easily penetrate the skin.
Consult with other tattoo artists about successful dark black ink tattoos that they have done. Ask what black ink they use and the method they used. By looking at various portfolios and talking to other experts in the field, you can make the black tattoos that you work on darker.