If the blob cannot do any harm 
to my freedom, or in other words, respects it, then that blob is acceptable.

 * For example, a hardwired boot blob which has been RE'd so that we know what 
it does and how it does it, would be acceptable (see Allwinner). Even the FSF, 
according to RMS's own essays considers this to essentially be hardware.

 * A non-ISA (a) firmware blob which controls a piece of hardware which
  i) can only do one thing
  ii) without compromising the security of the system
  iii) and is non-essential for the functioning of the system

is acceptable. Examples would be USB 3.0 firmware blobs. Examples of blobs 
which would NOT be are ME (violates all three points), MRC (violates point 
iii, and potentially ii).
 * An ISA blob which is NOT essential for the bring-up of the system, and can 
reasonably be replaced by a free alternative. This basically includes VGA 
BIOSes.