Just looked this up because I saw a used copy of 'Blood Rain' at a bookshop and was curious. From what I can gather, Michael Dibdin's 'Blood Rain' is the seventh novel in his Aurelio Zen series. So yes, it’s part of a longer series about the Italian detective, but it’s not a standalone book with a direct sequel bearing that same title.
If you finish 'Blood Rain' and want more Zen, you'd go to the next book in the series, which is 'And Then You Die'. The series order matters a bit for character development, but each book is a separate mystery. I read 'Blood Rain' out of order and still followed it, though knowing Zen’s history with his girlfriend Tania added some layers.
I’d say the series itself is the sequel, in a way.