Hi Diego,
I just tried to replicate the problem by configuring sites as you described, but the blog post worked fine in the child site and the root site.
The only thing I have different is that I have settings like this:
<add key="UseCultureOverride" value="true" />
<add key="site1culture" value="it-IT" />
<add key="site1uiculture" value="it-IT" />
<add key="site2culture" value="en-US" />
<add key="site2uiculture" value="en-US" />
but I don't think that is a factor in the problem. Culture is used for string formating of numbers and dates, and uiculture controls which resource files are used for labels and buttons.
Content workflow is not implemented in the blog so I don't see that as a factor.
Only thing I can think of to check is to make sure the time is correct on the server.
Hope that helps,
Joe