A great blog can be written about any subject you just have to pick one. A great blog should be short and sweet and get directly to the topic of the blog. If your blog is about pretty fall flowers, don't write about anything other than pretty fall flowers.
A great blog should include white space; it helps to draw the reader's eye and attracts attention to the page. Your blog should also be readable. Sometimes people choose a cool, busy background or a dark background for their blog, and then they choose a font that does not show up clearly and makes the blog hard to read, avoid this. Color and backgrounds are fine as long as they do not interfere with the visibility of the words written. If it is hard to read, most people will move on to something more pleasing to the eyes.
A great blog should contain words that are correctly spelled and used. All computers do have some type of spell check so use it. It is there for a reason. After using spell check you still have to proofread the blog, words can be spelled correctly but still wrongly used in the article. For example a common mistake is "you" and "your", you may have meant to write something like "your pretty fall flowers", but you forgot to put the "r" on the end so "you" is not a misspelled word but "you pretty fall flowers" is wrong and doesn't really make sense so always read your blog over a few times just to make sure it correctly written. Also, if you are unsure of the meaning of a word don't use it. There are plenty of other words you can use in it's place.
A great blog should also include links. These links can be to other topic related blog or articles, they can be to retailers and businesses, they can also be to other web sites. Creating links in your blog is important and if you join the affiliate program connected to the links it can also help to rake in some extra cash.
Keep these things in mind when writing your blog and it will be a great blog, hopefully read by many.