If you want the post to be visible on your blog, after you post it to the community you have to resteem it.
Note that this doesn't seem to be the case. As I said in my longer response, anything you post to a Community appears to be posted to people who follow you just as any other post would be. It appears right there in their blog. And this seems to be true whether or not they're reading you on SteemPeak or Steemit.
For example, this original post showed up in my feed and I am pretty sure that you posted it to a Community. I'm reading it on SteemPeak because I follow your account and I think it's guaranteed that I'm not in whatever Community you posted to. (I'm betting that the Community is owned by @hive-144703. SteemPeak doesn't automatically resolve that reference to a Community while beta Steemit does.)
At least at this point, for people who follow you as an entity, you don't have to do anything for people to be able to follow posts you make to Communities.
"Note that this doesn't seem to be the case." - that is because you are not using the beta site. the beta site is connected to the blockchain and steemit and other fount ends can pick up the posts. however, when beta changes to live, from what I understand, this will change.
As I said – "on SteemPeak."
At this point, the useful thing of looking at posts not on the beta Steemit site is that you can see exactly how things are working under the hood, how posts are propagated inside of the blockchain itself. Knowing that, we can make some assessments about what things are possible within the context of transactions issued via the blockchain.
But most importantly, other front ends are already picking up those posts. They aren't able to associate the Community owners with Community descriptors nor apply some sort of tagging or grouping to the content, but they do propagate it and allow it to be read, and that's how we know exactly that a post to a Community is treated just like any other post that an account would make.
In the longer run, this could be a bit of a problem – but it's part and parcel of the fact that the entirety of the blockchain is readable by any observer who wants to decode it. Closed/private Communities are literally impossible. The only kind of access control that will be available is determining who can post content effectively cosigned by the Community as an authority. I think a lot of people may end up being somewhat disappointed to discover they won't be able to, for example, set up a Community with a paywall for reading that content.
It's going to be very interesting to see what people actually end up doing with Communities. I'm actually quite surprised at this point that SteemPeak hasn't put up at least a beta interface to Communities considering how fast they jumped onto Tribes, which have a lot more programming overhead to make work. I trust the guys it SP to have a much better iterative process once they get something up in the first place.