I think the 'perfect one-brand-suits all black trousers' is very difficult to find - what one person thinks are 'perfect' another person may think differently - say due to different body shapes. I see you are in England, to be honest, my 'perfect' black trousers come from M&S! They seem to be the only brand that fit my body shape perfectly. I always 'stock-up' when I'm there. I know they may have a reputation of 'frumpy' but if you choose carefully, with a nice top and heels, no-one would know!
I don't think there is really an answer, no pair of trousers suits every wearer. I suggest a good old shopping trip and trying on, take pictures so you can see as well and look back at them, and take a friend to help you decide