Are there any rules for adding comments to blogs?

Yes!  In fact, there are.  In real life we have what’s called etiquette.  Of course you can guess the online word: netiquette!  There are special guidelines for adding comments to blogs.  First and foremost, know that adding comments to blogs is appreciated–especially since the blogger usually writes for a dialogue to occur.  Not all blogs need comments, but you could guess that all comments are appreciated.  But, it’s important to follow the same rules you would in person.  Let’s call them the three p’s: be polite, be positive, and be poignant.  That last one means you should always comment with a point in mind. 

If you want more reliable guidelines for commenting on blogs, you should definitely read the Lifehacker’s tips.