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James Blish resembles Poul Anderson in quality though not in quantity.

Volume I of Blish's After Such Knowledge Trilogy, Doctor Mirabilis, is historical fiction without any element of fantasy. Anderson wrote five such volumes. Doctor Mirabilis has one element of ambiguity. Maybe Roger Bacon's inner voice is that of a demon - but a fantasy would state that it was a demon.

In Volume II, Black Easter/The Day After Judgment, demons are real. Thus, this volume is fantasy. In fact, these demons are fallen angels, not powerful beings from another universe, as in Anderson's Operation Chaos. However, Anderson wrote several volumes of fantasy involving undoubted supernatural beings.

Volume III, A Case Of Conscience, is not Blish's only sf novel about interstellar travel. However, Anderson also wrote considerably more volumes in that category. In A Case Of Conscience, as in Anderson's "The Problem of Pain," an encounter with extrasolar intelligences tests the faith of a Christian character.

The moral of this post is: if you like Anderson, don't miss Blish; if you like Blish, you will find many more works of the same genres in Anderson.