Window treatment light control is described with several overlapping-sounding terms that actually mean meaningfully different things.
Three real levels of light blocking
Sheer and light-filtering fabrics let a meaningful amount of light through while softening its harshness and adding privacy — the room stays bright, just less directly sun-glared. Room-darkening fabrics block most, but not all, light, usually still allowing a soft glow to come through and some light around the edges of the panel. Blackout fabrics are specifically constructed (usually a tightly woven fabric or a special lining) to block nearly all light, commonly used in bedrooms for sleep or in home theaters. These are genuinely different products, not just different marketing names for the same fabric weight — checking which specific level a product actually claims, rather than assuming "darkening" language always means blackout performance, avoids a real mismatch with expectations.
Curtains vs. blinds vs. shades
Curtains (fabric panels) add softness and warmth to a room's look and are relatively easy to open fully to let in maximum light, but generally offer less precise light control than hard window coverings. Blinds (horizontal or vertical slats) allow precise, adjustable control over both light and privacy by angling the slats, without needing to fully open or close the covering. Shades (a single fabric or material piece that raises and lowers, rather than slatted) offer a cleaner, more minimal look than blinds while still providing more precise control than typical curtains, though generally less adjustability than slatted blinds for fine-tuning light angle.
Width matters more than most people expect
For curtains specifically, panels that are too narrow for the window, or hung directly against the window frame without extra width, tend to look skimpy and let noticeable light leak around the edges even in blackout fabric. A common styling and functional recommendation is choosing curtain panels wide enough to fully cover the window with some extra width on each side once closed, both for a fuller look and to actually block light effectively at the edges.
The one thing people forget
Measure both the window itself and, separately, where the rod or mounting hardware will actually sit, since curtain rods are commonly installed higher and wider than the window frame itself for both a fuller visual look and genuinely better light-blocking coverage at the edges — measuring only the window opening itself can lead to curtains or hardware that don't actually fit the intended, wider mounting position.