Keys were 14 stitches wide with a purl row between. Knitted cast-on. Slipped-stitch selvedge.
Knit to appropriate height, then bind off 7 stitches for the black keys centered on the appropriate purl column.

When knitting the remainder of the white keys, add a selvage stitch for the slipped edge(s) as necessary.

Knit the black keys separately, 9 stitches (would probably be better 8). Slip the selvage edges.

To assemble, place the black key in one of the gaps and using white yarn slip-stitch around the edge of the black keys, picking up two legs of both of the selvage edges (three if you did 9 stitches with the black keys). I had breaks at the corners but it would be much nicer to not.

Once all the pieces are in, knit across the whole thing using white yarn, then bind-off

Details:
Slip-stitch crochet with 3.0mm hook up the purl bumps for the between-keys demarkation and at the edges of the piece. Slip-stitch along the bottom and top edge of the piece so that the chain of the black line is right next to the chain of the bind-off/cast-on.

Chain using white yarn along any of the looser places at the back.

Border:
Single crochet into the front leg of the selvage/bind-off/cast-on edge for the border with the back leg in front (to form the white detailing), picking up 3 for every 2 chain (every four rows) on the sides. Add 2 stitches at the corners.

Second round hdc around, adding one extra stitch in each of the 5 at the corner.

Dots: Using larger needles, Magic-circle start with 6 stitches. Staggered the location of the increases
sc inc into every stitch (12)
sc inc, sc around (18)
sc inc, sc2 around (24)
sc in, sc3 around(30)

last stitch slip-stitch into first sc of previous round.
sc around in back loop, ending in slip-stitch (30, round-start shifted by 1). This will be the edge row.
Erm, some fancy stuff to get the edges to line up.
sc around in back loop, decreasing every 5 stitches.
sc through both loops, decreasing every 4…
etc. When 6 stitches remain, cut yarn, weave in and cinch closed, done.

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Actual measurements:

• white keys 6” by 7/8”
• black keys 3 1/2” by 3/8”

Keyboard:

• white keys 3 1/4” wide, so 3.7 times bigger
• 22” deep, 9 1/4” to black
• black keys 13” deep, 1 3/8” wide

Swatch:

• white keys 1 5/8”, 11” deep, 4 5/8” to black
• black keys 6 1/2”, 5 1/16” wide