I would make a simple garter stitch border in black.
Work one side at a time, please....not around and around.
("around and around" is usually always a hideous disaster!)
RS facing: pick up 1 st in each stitch
WS Row 1: knit
RS Row 2: inc 1 st in first st, knit across, inc 1 in last st
Repeat Rows 1 & 2 until the border is the 'height' you like.
Bind off on a Row 1, WS facing. (important)
The increases on even-numbered rows creates a "mitre" for the corners to be seamed together (like a picture frame) when you are all done. You can weave them together invisibly.
Here is a photo of my LOG CABIN with that border:
Here's another LOG CABIN I made for my granddaughter using Stash yarns:
It's a really nice, neat, clean, tailored edging!
Your blanket is beautiful, and very tailored looking.
A border such as I've described will visually 'suit' your blanket very well!
(No Crocheting: ack!)
After picking up stitches and working the border for SIDE 1 (the narrower end)...if you think it is rippling...then tear it out...and pick up 2 border stitches for every 3 stitches on the blanket. This will prevent rippling. (same formula as used for picking up stitches for buttonhole bands).
Don't hesitate to tear it out and get it right! Your blanket is too nice to have a ripply border! It deserves a great "frame" around it!
My suggestion: make the border a bit wider than the stripes are.
It will "frame it" much better, since the stripes are a powerful feature.