If I 'publish' something online -as a blog article, or pdf- will it be less attractive to publishers?
Yes, it will be less attractive to (most) publishers.

Most publishers operate as businesses and seek to maximise profits [citation needed]. The availability of an earlier edition on the Internet will reduce the demand for the publisher's version and thus the potential revenue that could be gained from publishing your book.

The extent to which your self-published version could impact on a publisher's revenue (and therefore their willingness to publish) will relate to how readily available it is and how similar it is to the version that the publisher would produce. If it is stored in a dark corner of the Internet and only loosely resembles the newer version then it may have little influence on their willingness to publish.

However, there are some publishers that operate on a not-for-profit basis; for example Open Book Publishers. These kinds of publishers probably would not mind the reduced profitability of your book due to previous publication.