The blogcategory option is simply a different way of displaying your existing categories - it is a display interface. There is no seperate entry just for blogcategories.
What happens is if decide to have a blog category menu link, it will ask you to select a category which to blog.
The blog category displays your news items like the frontpage, it shows the Title, Intro & a read more link. This gives a preview of the news items.
A content category on the otherhand simply lists the titles of the news items in your category.
Best suggestion is just to create one and see whether you like what it looks like. You can always delete it afterwards (deleting the menu link will not delete your content).
Have you read the
Offical Mambo 4.5 Manual?