This is a first blog post on this site after quite a long time…
Okay, but what does that mean? What makes "a first blog post" different to "a blog post".
That there haven't been another blog post "on this site after quite a long time…".
Well, we already have a well-recognised way of saying that; "This is the first blog post…".
That it's one of a few after this period? We already have the well-recognised; "This is one of the first…"
First has, in this adjectival position, a few well-understood meanings and here it is hard to see how it could be anything other than definitive and hence involve a definitive article.
Most writers will not get a large advance for a first novel.
Here the context is one where we are talking about a plurality of cases (writers' novels) and talking about what for each of those cases is the first (a given writer's first novel) applied to that plurality. So while for any given writer only one is "the first", across all of them we have many firsts, and so "a first novel" can make sense.
There are also noun senses where the indefinite article can easily apply:
Well, that's a first!