Either one will work...it depends on the look you want.

Donna

i prefer B

I use method B - I do not like exposed seams on the sides or bottom.

If the curtain is a solid color, the more invisible technique would likely look better. If it's a print, the outside stitching might disappear. I’m currently shortening good ready made drapes and the stitching is on the outside, it isn’t really noticeable with a matching color.

B ... looks more tailored and professionally done.

Drapery workrooms generally do side hems with the lining tucked in. They are stitched either by hand or with a blind hemmer - no seam running down the side. This keeps the lining from rolling out. Home sewing machines also have a blind stitch, but I wouldn't recommend using it with blackout lining. As mentioned above, be careful ironing, some blackout linings don't like a lot of heat.

I prefer the side hem and do it by hand because I don't care for my machine's blind hemmer stitch. For your orientation, side hem is on the left side.