What is the etiquette around blogging about conference talks?

My general rule of thumb in these matters is the following. If you don't want to communicate your results yet, you should not be talking about them in a public seminar. Or, at the very least, you should put out a disclaimer initially that you will be discussing confidential material (I have seen some company talks that started with a similar disclaimer, but probably more for legal reasons than because the presenter themselves cared deeply about it). Barring such a disclaimer, I think it's a fair default assumption that the presenter does not mind their thoughts spreading.

Another safety line you can take is, after you published your blog, making the speaker aware of it. I have found that Twitter works very well and non-intrusively to that extend:

.@coolprof summarizes topic X, feels more research is needed on Y. Summary: http://my.blog.edu

In that case you can assume that if the speaker really does not want your summary, or parts of it, on the Web, they will tell you.

Biology/paleontology talks can be particularly difficult to blog about if the presenter is introducing a new taxon name, because priority is attached to whoever uses the name first. If the name is not introduced in the print abstract before your blog post goes up, your blog post could become the "authority" for the name.

This is rare and people will understand what is meant, but taxonomy is a by-the-book discipline because there needs to be a way to make sure everyone is working from the same definition of a new organism description. As such, most people will avoid introducing new names in abstracts rather than peer-reviewed papers, but it has happened in the past.