Remember the source engine is still quake 1, it cant deal with a huge area like blackwood...
It's still the bsp engine from John Carmack and it's main idea is to use the bsp system, which means the whole "world" is split into vis leafs, and you can only look into another under special circumstances.
This worked nicely for small maps with many corridors like in quake, but it sucks as soon as you want to have a landscape.
And of course the maximum size of a vis leaf is limited, Im sure something like blackwood will need like 100 times more space or so.