Both Blish and Anderson:
were serious hard sf writers;
were among the authors whose series were edited by John W Campbell;
felt obliged to present new rationales for standard sf props like FTL;
wrote future histories;
speculated about the future of society;
were agnostics who took religion seriously;
also wrote historical fiction and fantasy.
Differences:
Anderson was prolific, able to produce many works very quickly, whereas Blish's much smaller body of work was written at times, we understand, with considerable effort;
Anderson not only wrote both historical fiction and sf but also synthesised them in several works;
Blish managed to combine the three genres of historical fiction, fantasy and sf in a single trilogy;
Blish's sf series are more closely interconnected as he develops related ideas in different directions.
