whats the best way to make this in buttercream.. blue w. some black in it??

I would use americolor navy. If you don't have that, use royal blue with a touch of black and a touch of violet until you reach the color you are wanting. Help that helps!

I would take a styrofoam plate and put dabs of the colorings you are using on it and then take about a tablespoon of your icin gand put it in the plate Using your palatte knife mix small amounts of the different colors in to that small amount of icing until you get your desired color remembering that it will darken yet when it sets. Keep in mind or write down what color combo you used. Cover with seran wrap and let set then check it in an hour or so. I find that if I use this technique I don't "ruin" a bunch of icing trying to get new color combos when trying to color match for weddings. You can always start back over with another tablespoon of buttercream if the color you get is way off HTH