It's not that Intel graphics cannot run OpenGL or DirectX (thats kind of the whole purpose of integrated graphics)
Its that the integrated graphics can't compile certain shaders, so Blockland's shaders aren't compiled and the game can't use them.
It mainly depends on what Intel CPU you have, some can support newer versions of Shader Model and (alot of them) can only support a version that happens to be below Blockland's version, or just doesnt support shaders at all.
EDIT:
Erm I put in OP that I use a GTX 670...Not exactly sure where people are grabbing intel from..
I'm not sure why it isnt working on Mac then, could you post a console.log?
I think I've seen another person having shaders on mac.