The first step in this article helped:
[url=http://]www.dart-creations.com/joomla/joomla-tutorials/joomla-blank-page.html[/url]
Now I get an error message so can probably fix the problem...
Strict Standards: Declaration of JoomBlogControllerSettings::cancel() should be compatible with JControllerForm::cancel($key = NULL) in administrator/components/com_joomblog/controllers/settings.php on line 15
Fatal error: Declaration of JoomBlogTableSettings::store() must be compatible with JTableInterface::store($updateNulls = false) in /administrator/components/com_joomblog/tables/settings.php on line 100
I found these files and edited them, putting the code in the brackets like the error message says.. the last one had two places - actually line 100 was the closing bracket of the function. Now everything working OK!