Blackout is the most misunderstood word in this business. People order a blackout shade, install it, and are disappointed the first morning. The fabric is almost always doing exactly what it promised. The problem is somewhere else.
Blackout describes the fabric, not the room
This is the most useful thing I can tell anyone shopping for a dark bedroom. Blackout is a rating on a piece of material. It means light does not pass through that material. It says nothing about light coming in around it.
Hold a blackout sample up to a bright window and you will see nothing through it. That part is real. But a shade is not a wall. It hangs in or over an opening, and there are gaps at the sides, at the top, and at the bottom. Light finds every one of them.
Once you understand that, the problem gets much easier to solve, because now you are working on the gaps instead of blaming the fabric.
Where the light actually gets in
The side gaps. An inside mounted shade has to be built slightly narrower than the opening so it can move without scraping. That clearance is usually about a quarter to a half inch per side. It is a thin strip, but with a street light or a sunrise behind it, a thin strip is plenty bright.
The top and the headrail. Light comes over the top of the shade and through the space around the roller or headrail. On a roller shade the fabric hangs off the front of the tube, so there is a natural gap behind it unless a cassette or a valance closes it off.
The bottom. Where the bottom rail meets the sill there is almost always a line of light, especially if the sill is not perfectly level. Older Kentucky homes are rarely perfectly square. That is not a defect, it is just how houses settle.
Pinholes. On corded blackout cellular shades the lift cords pass through the fabric. Manufacturers have gotten very good at minimizing this, and cordless lift avoids it entirely, which is one more reason I push cordless in bedrooms.
Outside mount is the biggest single improvement
If darkness is the goal, mount the shade outside the opening, on the wall or the trim, and build it larger than the window. Wider by two or three inches on each side, and taller above the opening.
That one change does more than any fabric upgrade. Instead of a gap at each side you have fabric overlapping the wall. Light has to travel a longer, more crooked path to get in, and most of it does not make it. It is usually cheaper than adding specialty hardware to an inside mount, too.
The tradeoff is the look. An outside mount covers the trim, so if you have beautiful casing you give up some of it. In a nursery at two in the morning, most people decide they can live with that.
Side channels, and how far they go
Side channels are aluminum tracks mounted on each side of the window that the edges of the shade run inside. The fabric is captured in the channel, so the side gap closes almost completely. Paired with a cassette that closes the headrail gap, this is the closest a shade alone can get to a genuinely dark room.
They are not free and they are not invisible. You will see the channels on the sides of the window. They add cost, and they add care at installation because the channels must be plumb or the shade will bind. For a media room or somebody who sleeps during the day, they are worth every dollar. For a guest room used a few times a year, they usually are not.
Layering drapery over the shade
The other honest answer is drapery. A blackout lined panel on a rod that extends well past the window, closed over a blackout shade, gets you very close to dark without specialty hardware.
Drapery covers the side gaps because the panels overlap the wall and each other. It also absorbs sound, which people notice more than they expect in a media room or a nursery. A ceiling mounted track, or a return rod that carries the fabric back to the wall, closes the last light path along the edge. We cover the hardware side of this on our drapery installation page.
The tradeoffs are bulk and cost. Drapery takes wall space, it needs cleaning now and then, and it is another purchase on top of the shade. It is also the most forgiving option, because soft fabric hides imperfections a rigid shade cannot.
What to realistically expect
Here is the plain version. An inside mounted blackout shade by itself makes a room dim, not dark. That is good for sleeping and not good for a projector. An outside mounted blackout shade, sized generously, gets you meaningfully darker. Add side channels and a closed headrail and you are in the range most people mean when they say blackout. Put drapery over that and you are as dark as a residential window gets.
I would rather say that before you order than after. Nobody is unhappy with a shade that does exactly what they were told it would do.
Matching the approach to the room
For a nursery, I usually start with an outside mounted blackout cellular shade. Cellular adds insulation, which helps keep the room comfortable, and cordless keeps cords away from a crib. Our honeycomb shades page covers the opacity choices.
For a main bedroom, an outside mounted blackout roller or cellular shade is usually enough. Drapery gets added when the room faces a street light or takes a direct east facing sunrise.
For a media room, channels or drapery are not optional if picture quality is a priority. Any stray light lands on the screen and washes out the contrast.
For someone who works nights and sleeps during the day, I go straight to the full stack. Outside mount, oversized, channels or drapery, and a fabric with a solid blackout backing. Daylight is far stronger than anything you fight at night, and half measures will disappoint.
Decide it at the window, not from a catalog
Every one of these choices depends on the actual window, the actual trim, and what is outside it. I measure with a laser, look at where the light is coming from, and give you a straight answer about how dark that room will get. If you want to see blackout samples held up in the room they are going in, book a free in-home consultation and I will bring them out.