Do Roman blinds keep light out? Yes, but the extent to which they do this will depend on what they’re made of. Roman blinds that are not specifically made as blackout blinds are made of soft fabrics and linings, which tend to be fairly thick but that can vary in terms of how much light they let in.
As Roman blinds are generally fairly high-end offerings, the quality and thickness of the fabric itself, plus the fact that Roman blinds are always lined, means they tend to keep more light out than, say, a roller blind without a blackout lining can do. Roman blinds keep light out to a reasonable degree when fully closed even when made of regular fabrics rather than blackout ones; they certainly dim rooms to the extent that many people find perfectly acceptable to sleep in.
As a broad comparison, Roman blinds keep the light out to around the same degree as a pair of curtains made of the same or similar fabrics and lining materials.
Do Roman blinds keep light out entirely if you choose blackout linings?
If you buy blackout Roman blinds, these will prevent all light from passing through the fabric part of the blind itself. However, as is the case with regular Roman blinds, you may still get a little light leakage around the sides of the blind, which may be a problem if achieving a full blackout effect is really important to you.
How do I stop the light coming through the sides of my Roman blinds?
Moving on from that then, if you have blackout Roman blinds or would be happy with the dimout effect of a regular Roman blinds were it not for the light escaping between the edges of the blinds and the window itself… How do I stop the light coming through the sides of my Roman blinds?
You can mitigate light leakage to a reasonable degree by fitting your blind outside of (over) the window recess rather than inside of it; which does mean that the blind needs to be a shade larger than the window recess itself, and cover a little more of the wall it is on.
The wider the margins where the blind extends over the wall, the less light leakage will occur.
The other alternative if keeping light out is really integral to your happiness is to fit a blackout Roman blind inside of the window recess, plus a pair of blackout curtains over the window recess, which assuming that both blinds and curtains are fully closed, will block out virtually all external light entirely.