Yes, that is what I would have thought about Muddy until I spoke to an angler along that stream last fall, who has been fishing a particular downstream stretch for years. So perhaps in that area conditions depend upon the summer in question.
Per Sarce's observation, it pays to carry a thermometer, as some Class A streams may even get warm under certain conditions. That appears to be a rarity. My surveys of wild trout streams, and not just Class A's, for 41 yrs have rarely found warm temps, suggesting that there are many wild trout streams that can be fished without temperature concerns. Blymire is not very surprising, however, as its development of a good wild trout population has apparently been relatively recent, so given Sarce's report it may still be subject to marginal conditions at times. In addition, its headwaters may still not be wooded (if memory serves it was agricultural-early succession years ago) unlike many wild trout streams.