My second floor deck blocks the light to the first floor. I was thinking about a plexiglass or glass block deck. Any suggestions? (You are correct to assume I don't know what I'm doing.) Thanks, Amy

Not the first person to say that. But if you do not ask you will never know if there is an answer.
No there is no such thing unless you have tons of cash as far as a clear plastic or glass deck for residential use.
What we have done in the past for clients to lighten up the lower room area was to cut any shrubs down around the edge of the deck on the ground, and place a ligher floor color on the cement slab if you have one below the deck. If you have any trees with a large canopy they also need to be thined out to allow more natural light around the edge of the deck. If you do not have any patio below consider installing one or pour white marble stone for reflection use.
Use brightly colored decorations that reflect light along the edge to bring in the light into the room. These can be white plastic chairs, tables etc. These need to be placed along the edge to reflect light into the dark areas below the deck.
If you deck is rather large, you can build a skylight of sorts with railing somewere in the middle.
If its real dark, lighting between the beams can be used to provide faux lighting during the darker days.