Oh I don't know, I think a darkish royal blue would look very nice, and you could have accessories in another colour. My mother wore navy and cream (lots more navy than cream) at my sister's wedding and dark brown to mine. She didn't look funereal or dowdy on either occasion.
greydog do you have the bride's mum's contact details? Perhaps your DIL-to-be is super busy and hasn't really focussed on the colours of her mum's outfit yet. If you could contact her Mum direct, and tactfully say that you'd like to make sure you don't clash/imitate her outfit, you might win some brownie points towards the "Considerate MIL of the year" award. I don't recommend going through your son (unless it's gets late in the year and you still haven't heard) as he might not phrase it properly and you end up coming across as "Me - what shall I wear?" instead of the thoughtful person you are.
Midnight blue is almost black. Weddings are happy, joyous occasions and my opinion is that the ladies' clothing should reflect that. Are there some colours that you look really good in, not everyday work colours, that you could think about it? It's your son's wedding - push the boat out as far as you can afford, and buy something "wedding-y"!
No, you can't wear all black - people will think either a) you can't be bothered and are wearing a work suit or b) you don't approve.
Get two jackets made to go with the black dress/skirt of the suit - one in black, one in a colour that goes with the bridesmaids, then you'll be able to mix and match later. Or buy something colourful on ebay and you can sell it on again after for much the same price.
Hope this is the right place to ask this! I'm the grooms mum at a wedding at the end of the year, and I have no idea what to wear. I have seen a beautiful suit pattern, which I would get a tailor friend to make up, but I would prefer to have it in black - this is because I do wear black a lot and nor for any other reason! I have asked the bride what her mum is wearing, and I've asked her to contact me more than once, but no one is coming back to me. I was really looking forward to this wedding, but now I'm beginning to - well, not exactly dread it - but the shine has gone off it. I don't want to be the Mother in law from hell, but I'm just at a loss. So, briefly, ladies, can I wear black?