Blood Music is built around a great science fiction concept: a man-made virus becomes sentient and starts rebuilding the world to their own specifications. (Yes, I know that they're technically lymphocytes, but they act and are treated much like a virus throughout.) And to start with, that concept is indeed very promising. The first half or so of the book seemed to be fairly hard SF to me. There are some issues dragging down the book as a whole, though.
The most immediately obvious thing is that the characters are one dimensional sketches, even the viewpoint characters. Nobody feels authentic, or even sympathetic. Adding to that, the narrative makes an abrupt turn towards outright fantasy towards the end. Sure, parts of the book up to that point unrealistic, but I feel fine with labeling some of the things that happen in the last few chapters as magic, pure and simple. Including the odd concept that the universe obligingly reshapes itself to the whims of whoever has the best current theory. It's a far cry from the original premise of the novel.
There's that great premise, though. And some really creepy imagery once the lymphocytes start relandscaping. It sounds very much like what might happen if Dr. Seuss and H.R. Giger were hired to design sets for a Tim Burton movie. Certainly worthwhile for SF buffs, but not something I'd give to somebody who isn't already a fan of the genre.