Publishing referee reports I have written - Yes or No and How?
Take care with this. Since you write them, the words are yours unless the journal has required that you yield all rights to them or otherwise restricts your actions. 

However, you should consider whether it is ethical to break the blind or double blind nature of review. There are reasons for that, though some are a bit esoteric. 

If you need to say negative things about someone's paper and they take it personally, then you could find yourself in the middle of an academic war and those other people with an interest in an author's work might join the fray. 

You also need to be sure that whatever you need to quote in a review is properly cited as in any other work. This makes publishing a double-blind review impossible, of course. 

Some things might need to be confidential for various reasons, including patentable work. Some things need to be reserved until the paper is published, I think. 

However, maybe the biggest issue is as follows. Often enough, by the time a paper is edited and re-edited, it no longer bears a lot of similarity to the version that you reviewed. In some fields this is a bigger problem than others, of course, but don't discount it. This will, potentially, reflect badly on yourself if you say things about a paper that aren't true about the published version.