They don't prevent all carbohydrates from being absorbed. They only actually prevent a portion of carbs from being absorbed. This means that whilst they are of some benefit, they're not magic pills that will shred all the weight off of you in a couple of months.
They don't directly block the carbs themselves technically. They just block the enzymes responsible for digesting these carbs. Some of them can block up to 65% of these enzymes. Which can in turn only manifest itself in 7% of carbs being blocked in total.
Do They Suppress Appetite?
Some evidence does indeed suggest that carb blockers can help decrease feelings of hunger and increase satiation. Rat studies have clearly indicated a decrease in food intake when rats were actively treated with carbohydrate blocking compounds.
Are They Safe?
Carb blockers aren't particularly well known for being unsafe. The general consensus appears to be that they are safe as long as you purchase from a trusted and reliable manufacturer. You should also take care to consume responsibly and follow any usage instructions as accurately as possible.
Any Other Benefits?
There are a handful of other reasons you might consider the use of carb blockers. One of which is taht they actively increase the amount of resistant startch present in your large intestine. Resistant starch is commonly associated with healthier gut bacteria levels. It's also associated with good blood sugar control and decreased levels of body fat. Some evidence suggests a link between good levels of resistant startch and improved insulin sensitivity. More research is currently needed to fully access the variety of potential benefits associated with carb blockers.