You can get them through ent_getmesh, using the level_ent pointer (or just NULL) for level blocks:

http://manual.3dgamestudio.net/ent_mesh.htm

From the mesh you can extract the vertex and triangle buffers, and get the coordinates. Be aware that y and z are swapped.

LPD3DXMESH pMesh = ent_getmesh(…);
if (!pMesh) { // no more blocks… }
BYTEPTR pVB, pIB;
pMesh->LockVertexBuffer(0,&pVB); // pVB contains the vertices in D3DVERTEX format.
pMesh->LockIndexBuffer(0,&pIB); // pIB contains the triangle indices, 3 shorts per triangle.