There is no reason why you cannot as it is body art quality which is the same quality for hair, eyebrows or beard as it can only be one henna powder, although there are cheaper less sifted (and altered with chemicals) henna on the market. Unfortunately to get black will be the same process as hair but if eyebrows are already black then indigo only can be used to darken brows (10-15 mins and test otherwise brows might get jet black). Having said that, it depends on existing colour of eyebrows and intending colour i.e. nearly black using 90% indigo to 10% henna, light brown 70% henna to 30% indigo, dark brown 70% indigo to 30% henna and medium brown 50% each. Dye release henna as usual and add indigo to henna. Be careful to wipe off the excess from surrounding skin area before leaving to develop. Best to use an eyebrow stencils/shaper which would make the job a lot easier and better.