This quilt I would (unfortunately) do the black and grey. But I might throw in one or two of the colorful construction blocks as you know babies love colors and different patterns, and there are research studies proving this. I would reconsider asking people about preferences as far as colors.....I do agree that asking if they prefer a blanket or bag is a great idea, but by taking out the color option this allows you, the creator, to express yourself and your colors to make the quilt as lovely as the infant that will be wrapped in it. I don't like seeing a small baby in black and grey it seems so sinister or bland. Babies deserve bright happy colors and designs.
Good luck. I know this would be so hard for me to do but I know you can muster the courage and create something wonderful and special for your newest family member.