I usually keep a size 16 in my machine and just change it depending on face fabrics, but you could use a 14 or even a 12, too. If you're using the older, regular type blackout, either spray some silicon spray or sprinkle some corn starch on it, or it tends to grab.
The one with the rubbery side is blackout and the one with the sueded type is dimout (insulating but not total blackout). Either can be used depending if you need/want total darkness or not.

I made a blackout liner for our family room drapes last October or so. We were having a heck of a time with sunlight and shadows on the television screen. Since this needed to be a panel more than 100 inches wide and 80 inches long, I had to sew a seam down the center. I believe I used a 16 needle. Then I went all the way around all 4 sides with my serger, but those had 14 needles in them. BTW, my blackout liner is white on both sides.

I didn't bother to hook mine to the drapery panels. Instead, I sewed buttonholes near the top of the liner, and hooked it over the drapery hooks.

DonnaR/CA