Hey @wobber -
noticing that this is your first post here and this is awesome.
Welcome!
I saw that your questions are about things which are not really specified yet which is awesome too!
The implementation of groups is very actively discussed here.
Check the Search feature in the top bar.
There is this active (very long 
) thread:
and maybe “Group Conversation Viewing” answers some questions …
The usage of Groups is not really specified in ActivityPub Core and that is where Fediverse Enhancement Proposals come in.
See Standardizing on ActivityPub Groups - #54 by grishka who created the FEP
a short hint: Markdown is supported here.
Personal flavour 
suggesting to format AS-vocabulary as preformatted text, which is also the code-button or the text betweent ``-tickmarks, so it is
Create instead Create
And then you are absolutely right:
but it seems, like only actors have an inbox to receive messages.
…
I think the best fitting type would be Group.
Am I right? Or is there a better fitting type?
It is the best fitting type, but ActivityPub is JSON-LD and type (like most properties in ActivityPub) can be multiple, so it is up to you to specify it further with other vocabularies …
E.g.
{ "type": ["Group", "https://schema.org/Blog"] }
The next question is: Should people read the inbox of a group’s blog or it’s outbox?
Or can the inbox and the outbox be the same object?
See this specification part for the differences …
They can’t be the same cause access control.
People read the Outbox …
Should I copy an Article from the group’s blog inbox
I guess if the Group inbox receives an Announce activity it is not specified, either the blog is an actor itself and if it is addressed, then “send the article only to the group’s blog followers list” …
Then see ActivityPub
and please ask again if anything is unclear …