Part 1 - Assembly
Part 2 - Skip to 9m 50s for blackout out the box and improved with black felt

We needed semi-blackout for a baby room but I couldn't find many real world reviews, pictures or videos of the blackout capability of the perfect fit range that attach to your existing UPVC windows. The first part of this video looks at the assembly for absolute blind amateurs like me. Then at 9m50s onwards I test the blackout capability, first 'out of the box' and then after adding some black felt flocked backing (Fablon brand - no affiliation) to the frame to improve it substantially. We don't get direct sunlight on these blinds until about May so that isn't tested yet but I will update if time allows.

Bottom line: out of the box is just 'ok' on a cloudy day, adding the black felt improves things considerably but you could go further with DIY if you need to. For projector / AV room use where total Batcave darkness is needed I would consider these if you are on a budget, are happy to DIY it a bit and don't get direct sunlight on the window when you intend to use them. If you get direct sunlight on the windows and intend to use in a blackout AV / projector room you probably need to spend quite a bit more on a more specialist product or be prepared to hack it further with drapes that can attach over the top of these blinds maybe with Velcro or magnet strips to aid lightproofing.