We used B & Q's own mid-blue which is quite a strong, warm blue which looks great against white skirtings and woodwork. It is matt and stated it could also be used on woodwork. We were going to paint built in wardrobe doors and apply new handles in GS's bedroom. We were sceptical of own brand, but it did what it said on the tin and the walls and paintwork look great. It was on offer and a large tin cost under £20 which did the whole room. I'm sorry I can't remember the name now but you could have a look.
I used mineral mist at my mum's and this is quite a lot lighter, cooler shade. You are right, you do have to be careful with blue. If your room is light, then I recommend the B and Q one.
I've just had DS's room done in Thunderous Sky endurance matt. I think it is great - strong but not dark. White ceiling and woodwork. South facing window. His high sleeper bed, currently pine and in bits in the shed while he sleeps on a mattress on the floor and gets used to going to the loo at night, is going to be repainted in slate grey.