Referring to the link at:
http://www.linkwitzlab.com/index.htmLinkwitz Lab has an excellent analysis of rendering a recording for stereo playback. The SRA, stereo recording angle is discussed, and it is pointed out an SRA of approximately 70 or 75 degrees will render stereo playback for an idealized equal lateral triangular relationship between the listener and the left and right speakers.
Consistent with this, Dave correctly intimates there is a fixed relationship between the placement of the mic array to the sound source and the mutual angle between the mics to obtain the best sounding result that he readily recognizes. Additionally, I would submit that the polar pattern of the mics and their interaction to shape the sound field, i.e., the polar pattern of the array, which also determines the acceptance angle of the array, and therefore the distance of the array to the performance to achieve the desired SRA.
Unique to the Blumlein’s among coincident and near coincident arrays is the minimization of monophonic reverberation in the sound field. Dave also correctly points out that the fixed nature of the relationships he mentioned make the Blumlein difficult to obtain a good result unless the sound source is placed as necessary to conform to the Blumlein array narrow acceptance angle.
And I agree this is not practical or even desirable, as many recording venues with great sounding acoustic characteristics would be excluded. The acceptance angle could be made wider with the 4 cartioid Blumlein by adjusting the gain ratio of the forward facing to rearward facing mics. If it were adjusted so the rearward facing mics were completely muted, then a forward facing XY coincident array with a wider acceptance angle than the Blumlein remains.
To the extent that the acceptance angle can be widened enough to capture the whole performance and allow for some contribution by the rearward facing mics, the less the reverberant aspect of the environment will be rendered monophonically.