The best blackout blind fabric is both a question of taste and dependant on the types of blinds you are looking at. For any given fabric, there are different ratios of absorption, transmission, and reflection that make up the physical properties. A blackout fabric, as the name suggests, is a fabric that prevents any transmission of light through it. One key thing to remember is whilst the fabric itself will prevent light transmission, there will still be differing amounts of light that transgress the blind. Either around the sides which is common for most blinds or through the slats, louvres, holes and working mechanisms. Below are some typical blinds that are available with blackout fabrics and how well they work to provide efficient dim-out properties.
Roller blinds are one of the better-known types of blinds and are widely available; as suitable for a small domestic application as a large commercial example. Physically, they consist of a roller, wrapped in a fabric of your choice, that can be raised and lowered to provide coverage and shading. There are many different systems on the market, from very cheap cardboard tubes, the sort that you would readily find in a DIY store and cut down to size yourself, up to much larger applications where reinforced aluminium tubes are used. The wider the blind, the stronger the tube needs to be as any bending under tension will cause the fabric to ripple.
Control options are typically a manually controlled spring mechanism or what is known as a side-winder. These all have to conform to strict child safety laws in the UK when they are installed. Nowadays, more and more automated control options are becoming available, with battery-powered motors becoming one of the most sought-after solutions.
Roller blinds can be installed either inside or outside recess and there will be light around the edges that is able to enter the room. An inside recess can provide better dim-out, though due to the design of the brackets there will always be a gap. However, there is a solution available known as a zip blind. This is essentially a roller blind with side channels and a clever zip mechanism that will prevent almost all light transgression and provide a near fully blackout solution.
A roman blind is made from a different type of fabric, a soft fabric, one similar to what you would use for curtains and upholstery. Quite often these beautiful fabrics are not blackout themselves but can be made with a blackout lining to create the same effect. They can either be handmade or machine made for a more economical product, however even with a blackout lining you can get small amounts of pinhole lights through the fabric where it has been stitched. For this reason, a handmade gives you both better quality and eliminates these pinholes.
Similarly, to a standard roller blind, there will always be light seepage around the sides of the blind. However, due to the design, you can have a much smaller gap, particularly when installing on the inside of a recess such that you really can achieve a very good level of dim-out, even without any side channels as such.
Perfect fit blinds are a trademark product that has been around for almost 10 years now and was designed by Louvolite. They are available as either a roller blind, a pleated blind, or a Venetian blind style and due to the clever way they clip to the window beading, provide a good level of dim-out. This type of blind is often the only option, particularly on new-build properties where there is no space surrounding the window to install a traditional blind, without blocking the inwardly opening windows.
The other advantage of these is that they do not require any screws or drilling making them easy to install, and easy to remove too without causing any damage. The perfect fit roller blind has the same fabric options as a normal roller blind, and the pleated variety has a very similar range of fabrics and properties. These can, of course, be blackout fabrics or translucent fabrics.