Before about 1500, critical reviews of contemporaneous works tend to survive either through loose anecdotes or through testimony encountered in theoretical treatises (which is basically the only sort of publication dedicated entirely to prose about music before the modern printing industry gets going). So a treatise from the late 13th century by a person now known as Anonymous IV has a lot of critical remarks about Parisian music making in the early decades of that century. Likewise, Johannes Tinctoris has a lot of (negative) things say about his contemporaries in the 16th century, and a lot of praise to heap on those of the 15th.

Criticism and reflection really gets going, though, with the rise of large scale public performance venues. This mainly happens once opera gets going around the 17th century. Things like the Mercure de France in Paris and The World in London in the eighteenth century have plenty of reviews of works currently being produced. I'm currently researching multiple musical compositions on a single opera text in the eighteenth century, and have a running file of all contemporary criticism I've found of it here if you want to take a look: https://docs.google.com/document/d/1aEFCC1Jf8HGbcqoNcsnETdGgDgD0VHbe_79Y_WIHEVk/edit?usp=drivesdk

In the nineteenth century, there are whole journals dedicated to musical criticism that start to appear. Perhaps the most important of which is Allgemeine Musicalische Zeitung.

My advice would be to seek out "source" anthologies in music history, they'll contain a lot of critical reflections alongside other things like composer letters, Libretto prefaces, theory treatises, etc. The standard anthology is the Strunk's Source Readings in Music History, which you can probably check out of a library.

Dang. No Responses. Looks like an opportunity to do some original research then :)