Blackout curtains are a completely different league. When I first sewed them, I completely underestimated their weight. I chose regular curtain tape, the kind I use for other curtains, and quickly regretted it.

What went wrong?

The top edge started to stretch, the pleats were uneven, and the whole thing looked heavy and unsightly. After a few days, I knew it wasn’t the sewing, but the tape.
Blackout curtains need solid support.

What worked?

I started using tapes designed specifically for heavy fabrics. They are stiffer and have stronger strings. This keeps the pleats in check and prevents the curtain from “pulling” down.
Now I order these tapes from curtainmaker.uk because I know they’re tested specifically for blackouts.

Conclusion

There’s no room for compromise with heavy curtains. A good tape does all the work.