I do alot of machine embroidery, and applique work. I tend to not use black as it seems a bit harsh,I feel it can sometimes overpower my designs. If I don't outline with each different color (yes I too am a glutton for punishment) LOL... I will use a dark grey or dark brown color. They are less harsh and for me I like the end result better. I do like the suggestions of using the deep blue to outline too. I personally think your plaid fabric would be the best option of the three for sashing. I too feel the prints would take away from the applques.
Totally IMHO
And... your quilt is just darling!!