Reproduce as following steps:
1. add Blog portlet;
2. add a new user and join in liferay.com site;
3.go to control panel -> Roles,click action near Site Member,choose define permissions;
4. add permissions Add Entry and Access in Control Panel;
5.back to liferay.com and add
6. login in as the new user,go to control panel -> Blog;
There was not Add Blog Entry button.