No need to mix singular and plural. Most JEs just have curtain.

@@ -13,2 +13 @@
-<gloss>blackout curtains</gloss>
-<gloss>light-blocking curtains</gloss>
+<gloss>light-blocking curtain</gloss>

Some pictures of the lowest level, 遮光3級
https://www.nitori-net.jp/ec/product/5731922s/

Types of 遮光カーテン vs light-blocking percentage
https://bicklycarpet.co.jp/column/curtain/20190718-01/
完全：１００％
１級　９９．９９％
２級　９９．８％
３級　９９．４％

https://eow.alc.co.jp/search?q=遮光カーテン
a blackout curtain
a blackout drape〈米〉
a light-blocking curtain
light-tight curtain〈主に英〉
lightproof curtain
shade curtain
shading curtain

Blackout curtains, blackout drapes, block out curtains, shade curtain; light-reducing curtains
https://context.reverso.net/translation/japanese-english/遮光カーテン

https://www.moondreamwebstore.com/content/43-buying-guide-blackout-curtains
8 - What is the difference between a blackout curtain and a room darkening curtain?
A blackout curtain offers blackout effect up to 100% whole a room darkening curtain blocks out the light around 90% to 95%. Please see our comparison chart to understand which curtain offers the right light blocking effect for you.

Note: "curtains" pluralized intentionally here, as this is a common form. (カーテン is "curtain; curtains")

You can use 遮光 to describe lower levels of light blocking, but once you stick 遮光 to カーテン, it seems to imply pretty significant blockage.

Only question is whether the definitions of "blackout curtain" and "遮光カーテン" are completely overlapping.  "light-blocking curtain" allows a little flexibility there I suppose.

"opaque curtain" returns 9 hits on google... "lined curtain" describes one possible mechanism, but not the effect.

@@ -12,2 +12,3 @@
-<gloss>lined curtain</gloss>
-<gloss>opaque curtain</gloss>
+<gloss>blackout curtain</gloss>
+<gloss>blackout curtains</gloss>
+<gloss>light-blocking curtains</gloss>

Entry created