Johnny-there are old copies that were pressed on blue vinyl and mastered by Stan Ricker (who was the guy behind a lot of the original MoFis, among other records).
Amirm- there's a lot of stuff in the literature about whether the black vinyl, which originally came from the use of carbon black and was intended, among other things, to reduce static (as well as conceal plate out problems) created electromagnetic anomalies. When Classic Records switched to their 'Clarity' formulation (a shade of translucent green) before their demise, they were touting this as well. I'm not sure it makes that much difference- comparing a 45 RPM Tull Aqualung-same mastering- on Clarity v Quiex (or whatever the high-end formulation was that Classic used), there was very little noticeable difference in sound- the Clarity may have been a little quieter.
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