Message: Best dye to dye a silk/6% spandex dress dark black?  

The only dyes I like to use with spandex blends are cool water dyes, such as Procion MX dyes, along with soda ash as a dye fixative. These dyes work very well at a room temperature of 70°F, which will not damage the spandex. Spandex is a heat-sensitive fiber that is likely to lose its shape if subjected to high heat. Check the care instructions in your dress to see how warm it is okay to get the water. 

Most black Procion MX dyes will produce a color other than black when you use them on silk. This is because silk, as a protein fiber, reacts differently with the individual dye colors than cotton does, and the predicted colors are standardized for cotton.  You can order an MX black dye mixture from PRO Chemical & Dye, in Massachusetts, that is called "Silk Black" and is just what you are looking for.

Alternatively, you can use another fiber reactive dye, Dylon Permanent 12 Black, or, if you are in Europe or Australia, with black Dylon Machine Dye. This particular black dye is an unusual single-hue fiber reactive dye that is not a mixture of several different colors, as most black dyes are, so it will produce the same color on any natural fiber, including silk. The label on the package will tell you to use a higher temperature than the care label in your dress will permit, but the dye will work in warm water if you give it more time to react. Unlike all-purpose dyes such as Rit dye, it will work well at 120°F. The advantage of using Dylon Permanent Black 12 is that you can often find it locally in a crafts store or a sewing store; Joann's fabric stores in the US usually carry this dye.

For more information, please see my page "How to Dye Spandex", and possibly also my page "How can I dye clothing or fabric in the washing machine?".

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