Because this is long I'll present my proposal first and then the background.
Linking to an external blog means SE incurs some work and some liability, so we need to protect SE's interests as part of this. Based on the background below, here is my proposal for the "governance" side of a "blog" link in the supercollider or topbar (or any other "built-in" integration):
The blog must already exist and have a decent track record. If we're going to ask SE to do work, it shouldn't be for aspirations of a blog but only after the community has demonstrated the ability to actually produce one and keep it going. So there should be some threshold for both age and activity level. (That delay doesn't hurt visibility; when a blog starts there's a lot of meta/chat conversation about it on the site already.)
The blog must clearly link to its SE site.
The blog must clearly explain how to contribute (something like SF&F's page, or Worldbuilding's).
The request for the blog link must come from the site moderators (perhaps more than one). The request would include an explanation of how the blog is edited/policed. The moderators need to demonstrate to SE that the risk level is acceptably low.
SE (community managers, I presume) must be able to throw a "kill switch" that removes the blog link from the site.
If SE can kill the link I don't think they also need the ability (or want the responsibility) to edit the blog. SE's concern is protecting SE, and killing the link decouples a site from its blog. But if granting blog access to one or more CMs would smooth the process, we could add that requirement. I think an SE community should be willing to grant such access in exchange for a link from SE.)
I asked a related question about promoting new blog posts automatically. A big concern with an official-looking link, whether it's a "blog" link in the supercollider or an automatically-posted "new blog post" announcement, is that SE is seen to endorse what's on the other side of that link. So there need to be some guardrails. At least once in the past, the editors of an SE blog went off the rails and CMs needed to step in to get some stuff nuked, which I think was harder than they expected it to be.
Shog's core advice was: Find a way to put all of the governance in public. While he was talking about events in the Community Bulletin, I think the same concerns would apply (perhaps even more strongly) to a "blog" link. Shog wrote:
And I don't mean in a chatroom, or off-site somewhere, or tucked away in emails sent to employees; I mean front and center on the site itself, visibly cross-linked with anything that someone might see upon clicking through the bulletin.
If I'm a member of twistystraws.stackexchange.com and I see something on the bulletin board, I should expect that when I follow that link I won't be leaving my membership at the door; there should be a clear and obvious way for me to engage in whatever I happen to be looking at, a clear and obvious way for me to critique, contribute, praise or complain. And when I do any of that, it should be visible to other members of the community who've followed the same path.
By the same token, if I'm new to the site and end up following that link... I shouldn't end up in some dark back-alley full of scary-looking people and no way back to where I started. Even meta sites have consistent navigation and privileges.
This post led to a conversation in chat in which Shog raised some additional points:
There should be obvious instructions on how to contribute.
Your SE identity should carry over somehow. However, as far as I know no SE blog, past or present, has enabled this; on most blogs you can sign in with the same OpenID credential, but things like your SE reputation won't be visible on blogs. So I don't know what to do with that request/requirement; it seems technically infeasible. It also goes beyond what was done when SE hosted the blogs.
Another community manager, Adam Lear, raised an additional point (talking about linking to new blog posts from the community bulletin):
As for the policy aspect... the main concern I'd have here is that in case something goes wrong, you'd need an employee with developer access to fix the community bulletin for anything urgent.
You'd have to coordinate removing the post, and then (unless you want to wait an hour or however long the caching there is, I forget), find someone who knows how to force a refresh.
The same general concern applies to any linking: if a community's blog is used to post something inappropriate, SE needs a way to take down either the content or the link. A community could grant SE employees editorial access to blogs for just such a purpose, but in addition, if something explodes, it's a pretty safe bet that SE's going to want to take action to prevent it from happening again. This might mean moderators are held accountable for blog content, or it might mean that the "blog" link needs to be revocable, or both.