Everyone has had the experience of disliking a product and getting all worked up into a lather. "I must warn the people!" But while it's true that a negative review can be a valuable asset, there's also a good way to go about it and a not-so-good way.
There's a good way to do it.
Bad reasons to write a negative review include revenge ("that company will rue the day they messed with me!") and humor value (it's always easier to be funny when you're going negative). A good review should be balanced, fair, and even-handed. It should not be written when you are feeling at the height of emotion. If you must, then write it and delete it unpublished just to get the vitriol out of your system.
A well-written review will reflect well upon you, bring more traffic to your blog, and make you look like an interesting and intelligent person. A poorly-written review, especially a negative one, will have the opposite effect. It makes you look sloppy, childish and vengeful - not exactly the best qualities to attract and keep an audience.
The best way to keep yourself on track is to imagine that the creator/company/designer of the thing you're writing about is going to read your blog post. Because the chances are good that they will. You might think that your blog is too obscure for them to ever notice, but the almighty Google will bring them right to your doorstep. Most companies and individual creators have Google Alerts set up to email them whenever a new mention of their name or product appears anywhere online.
With this in mind, pretend that your best friend has made this thing, and has asked you for your honest input. You won't be doing your friend (or the world) any favors by ignoring the flaws. Honest, humane feedback is how we improve. And it's also what your readers are looking for, when they read your review.
On a more granule level, try to avoid being overly dramatic or making sweeping generalizations. ("This was the worst sandwich I've ever eaten IN MY LIFE.") They undercut your point by making you look childish and unsophisticated. If you can explain the problem without sounding like you're complaining uselessly, then you are on the right track. Taking a sandwich for example, "The bread sucked!" is a useless complaint. "The bread was spongy and dry" is a simple explanation of the problem.
Be as specific as possible, and don't forget to mention the positives as well!