Hi Marc,
I have the same issue like jufo (she asked in April 2014 I think): I don’t want portfolio pages to be shown alongside Blog posts.
I’ve tried to create a Portfolio section by following this advice: http://en.support.wordpress.com/portfolios/
It’s not working.

Do you have any other suggestion? You mention another plugin. Could you specify exactly what sort of plugin to use?
I’m also considering linking to a seperate Blog WP theme, FYI.

My site: http://sophielepinoy.com/portfolio-2/
on the Portfolio page, under the Brand Design section, I removed the 3 items that were in blog posts format by changing the slug in the Appearance/Widgets section.
I created a Portfolio page (http://sophielepinoy.com/portfolio/elisabeth-lang-interiors/), hoping it’d show under portfolio, but it’s not showing anywhere.

Thank you
Sophie