How much light do Vertical blinds block?
Blackout Vertical Blinds
Vertical blinds are very versatile they draw back, rotate 180 degrees, practically dust free great alternative to nets, I have personally fitted a great deal of them to bedrooms it is not uncommon for every room in the home to have a vertical blind.
What you must appreciate is yes a blackout fabric will block the light but remembering you will have light bleed on every slat overlap, across the top where the fabric is located to the headrail and along the bottom to the window sill, all blackout fabrics will emphasize any natural light that filters through / around the blind if there are gaps.
To be honest only the end user can answer the question of will a vertical blind block enough light to enable trouble free sleep, it is a very personal opinion so subjective I would say in answer to your question 70% light block due to the gaps.