Canonical block names: spaces vs. underscores

[Mathias Bynens]

`Blocks.txt` (http://unicode.org/Public/UNIDATA/Blocks.txt) lists blocks such as `Cyrillic Supplement`.

However, `PropertyValueAliases.txt` (http://unicode.org/Public/UNIDATA/PropertyValueAliases.txt) refers to this block as `Cyrillic_Supplement`, with an underscore instead of a space.

Which is it?

If proper canonical block names use spaces instead of underscores, why doesn’t `PropertyValueAliases.txt` reflect that? 
If proper canonical block names use underscores instead of spaces, why doesn’t `Blocks.txt` reflect that?