The blog by default disables TinyMCE and uses BBCode (not a smart idea and I dont like that). If your using a recent version of the blog - available off dailybuilds.silverstripe.com you can use call this in your mysite/_config.php file
Re the change to vaild_elements - if that doesn't work then I have no clue as thats the standard fix for tiny_mce. Make sure it hasn't cached the JS file by clearing your browser cache and visiting http://yoursite.com/admin?flush=1
However, DO NOT DELETE THE DATA AND ADD AN ASTERISK SINCE THAT WON'T WORK.
For example, if you would like to include the "style" attribute in a DIV, just add ", div[style]" at the end of "extended_valid_elements". More info can be found at: