Thanks. I don't think they are too small as they are 1.5 sizes bigger than my shoe size and the toes which are black are not the longest toes.
They are also the same make and model as my last ones, which gave me 1 x black toes and were half a size smaller.
I think Seren is right, I need to focus on the advice given rather than the shoes.
Is the lacing too tight then if my toes are going numb?
As for being happy with the shoes at the time, how can you tell if the shoes are going to give you black toes? It doesn't happen in 30 secs on a treadmill. They don't feel too tight, they feel fine. But when I run for over a mile I get black toes and numbness. The black toes I can live with, but the numbness cannot be good.
I do understand that the shop cannot resell them, but I can't use them, and I paid £90 for them, so I'm pretty unhappy.