When I create a new page in Blocs content from other pages appears on it.
Duplicated content on new pages usually means that some of your site’s content is located within the Global Area of the project. Global area content will appear on every page, it’s intended to be used for the navigation and footer of a site.
To Remedy this issue, simply move the content out of the Global Area using the layer tree or by nudging the Blocs with the keyboard shortcut ⌥⌘↑ to nudge up or ⌥⌘↓ to nudge down.
You can find out more about the project Global Areas and how they work here.