I love Ellen Kennon Slate blue. I had it in the bathroom before the start of the remodel, and I picked everything so it would match the same colour when I was through.
Sally

My master bedroom and bathroom are Bejamin Moore Buxton Blue.

I would go with a blue based gray. You'll get a calmer more spa like blue that way. I only have the SW color deck but SW shades that fit this description would be colors like Topsail, North Star and Krypton. If a darker blue is more of what you're looking for you may want to look at Sleepy Blue, Languid Blue or Rain

Love blue-grays. We recently chose BM Affinity color Tranquility (Aura) for our basement bath (which has no windows so no natural light). Lovely, soothing color. Crystalline is a touch paler and also beautiful. I'd posted some photos of Affinity blue-gray sample cards in my blog. I have the paint ready, and I'm planning to paint within a few days.

Here is a link that might be useful: Photos of Affinity blue-gray sample cards

I have a suggestion that isn't about your blue but rather your mirror. Since you are in frugal mode and painting your vanity anyway, you could buy molding to put on the edges of your mirror. You could then paint the molding the same color as your vanity. Or, you could paint it white to match your other trim.

I did much of my house in Dunn Edwards Sidewalk Chalk, which is about halfway between a baby blue and a Tiffany/spa/robins egg blue, with some gray. It matches very faded denim on a sofa, if that helps visualizing. It's a very calming color and would be nice in a bathroom. My camera tends to make everything purple, so if you're interested check it out elsewhere, but here's a pic:

This one is Benjamin Moore Whipple Blue. This is also a room with no window. I like that it's a deep dramatic color but still a soothing blue.

I just painted on a sample pot of BM Whipple Blue and Buxton Blue today. Heck, I don't know. Whipple Blue is too blue, Buxton Blue is too green. I'm sure whatever I do I'll like, I just can't decide what. :-/

Benjamin Moore has lots of great soft, blue-greys that would enhance a spa-like, updated atmosphere. I think you might want to try them out with the lighting in the room in order to make a satisfying decision.

Good luck and please let us see the final results. I like your frugal approach. I think it will look very nice.

Our kids' bath is BM Heather Gray (no windows). Cabinets painted black.
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And we are working on this bathroom now--still have to frame out mirror, etc. But it gives you the general idea. It has one window, which does affect the color outcome for sure. I tried several blues until I found this one. SW Upward on walls.
Painted the cabs SW Pure White.
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