Caveat: This is a review of this show after only 20 minutes of the first episode, so take it with a grain of salt. First, historical INaccuracy reigns throughout and is sooo annoying. The characters wear clothing, have hairstyles, and use idioms not invented yet. Also they do things such as open-mouthed kissing and clutching each other which in “decent” society in Victorian would be totally unacceptable. And yes, there’s the issue of the colour-blind casting. The Victorians were not colour-blind. On the contrary, they were exquisitely colour-aware, and enforced their racism. Why does this matter in a series about fighting black magic you ask? If it were a series about fighting black magic in mid-60s London and later, I would agree with you. It wouldn’t matter. But this series is trying to live as if it’s in the 1840s. So the chosen historical context IS front and centre. If this were set in 1960s London, then racial minorities, by that time, would be visible enough in the general community that the casting becomes appropriate.
The producers wanted to have their cake and eat it too. Pretending to be aHolmes story and bringing in the period atmospherics, while appealing to teenagers with their cloying romances and action sequences - oh and black magic.