Instead of depending on the Blogger Spell Check, I used to prefer the FireFox Google toolbar spell checker. However, since they introduced FireFox 2.0, that was not working and I resorted to using Microsoft WORD to do the Grammar and Spell Check.
All I need to do is to click inside the Blogger post editor window, press ctrl+A to highlight everything, press ctrl+C to copy everything into clipboard, open MS WORD, paste everything (ctrl+V) into a WORD file, click Tools > Spelling and Grammar Check and WORD will check both spelling plus grammar for you. When done, click inside the checked WORD document, press ctrl+A to highlight everything, press ctrl+C to copy, open NOTEPAD and paste everything (ctrl+V) into a NOTEPAD document, press ctrl+A and ctrl+C to copy everything in the NOTEPAD document to clipboard, go back to the Blogger post editor window where the original post was all highlighted, press the DELETE button to clear the post editor window, press ctrl+V to paste everything from NOTEPAD document into the post editor window. Publish.
Note 1: The purpose of going through NOTEPAD is to strip any formatting that might have been introduced by Microsoft WORD into the file.
Note 2: If WORD did not find any errors and thus no correction, skip the steps of copying into Notepad then into the Blogger post editor. Just publish direct and ignore all that had been done above. WORD Grammar and Spell Check did not make any correction for this post so this will just be publish direct.