Blanket squares

This might be a dumb question, but here goes…

I am making a baby blanket for a friend out of 5 different colors of blocks. How on earth do I seam these without it looking wonky? What color yarn would I use? I know I could do a sing;e crochet border around all squares in a different color, and then use that color to seam, but thats a lot more work than just sewing them together! I have my own 2 year ol and 3mo. old to contend with and knitting time is very sparese these days!

Mattress stitch is supposed to leave an invisible seam~!As for as the yarn what colors are you blanket~ would black work since it hides more?
Got any pics? :??