How do I make summoned falling_blocks look like a block other than sand?

So your command is just a bit malformed, from some tutorials and testing I have found you can do it as follows:

/summon minecraft:falling_block -261 39 -2426 {BlockState:{Name:"minecraft:magenta_carpet"}, NoGravity:1b, Time:1, DropItem:0b}

What is important to also note is that you also need the Time data, which states how many ticks the entity has. According to this Minecraft Wiki guide (as of this post's creation) it states than a value of 0 is a despawn, a value of 1 to 599 means the entity stays spawned, and a value of 600 is also a despawn. This means the entity can live at most 30 seconds before despawning, according to the guide.

What is also important to note is the DropItem:0b since this means you won't drop a carpet at the end of the 30 seconds.

If you use this method to generate a fake floor, you will need to use repeating command blocks or a clock attached to one to respawn it every 600 ticks.