Richie ones do not last as long as Carrs Billington ones. The latter will last happily from scanning to lambing and beyond - two months and more. They give a good solid blob of colour if you give it a good long press.
Richie do nice handy small tins, so very useful to keep by you for a temporary mark. But they wash out much easier than the Carrs ones, and you can't get such a good blob of colour no matter how long you press - so not as good for your black sheep.

Downland is between Richie and Carrs for longevity.
I haven't tried Marksman.
Rosemary, what brand of crayon do you use? I tried some, thinking they'd be handier in a pocket, but I couldn't ever get a mark you could see from more than 3 paces, and to get any kind of mark I would need to hold the sheep still while I scribbled on its bum - not always possible out in the field!
Back to the OP - I've no experience of marking black sheep but I'd be inclined to try Carrs' orange or yellow.