I think that the color schemes are totally mixed up when it comes to baby showers.
I love telling people how recent the idea of pink for girls/blue for boys is, because you can bet anyone who is a stickler for the concept has no idea that early 20th century boys wore pink because it was the more masculine color.
This has always always been one of my favorite facts! Correct me if I'm wrong, please, but wasn't that because blue represented purity and pink was derived from red (blood, battles, and all that jazz, some conditions may apply.)?
More on point, OP, the colors you chose do sound quite neutral and more than likely look lovely together; most stereotypical boy colors seem to be more bold and pigmented rather than "baby" or a pastel such as mint.
If you're afraid of soon-to-be-grandma-Meg's opinion, perhaps as suggested above, give the gift before the party? Personally, I might contact Melissa asking how the party is shaping up, if she needs any help, and mention how much I enjoyed making baby's present and how the time it took was SO worth it, but that I was a touch worried that the colors might not be "feminine enough" in the eyes of some of our family. If she agrees someone might put up a stink that it's not pink, maybe meet for a bite to eat and give her the blanket there, then pick up a smaller gift that is more run-of-the-mill for the actual party.