Whenever Dexter has his victim on the slab ready to be killed, he cuts them with a surgical knife and takes a blood sample. Obviously the knife is fake and the blood is fake but quite often you will see the knife touch the skin and the fake blood comes out, emulating a cut on the cheek.
How is this effect done?
It's a hollow knife - fake blood is pumped through and ejected when the fake blade touches the skin.
Or the blood is routed along the (very blunt) blade on the side away from the camera. If you're a right-handed actor, you can use this knife to slit someone's throat without the pipework and bladder being visible to the camera.
This is the most effective/realistic method.
Or you can use prosthetics and have a real knife slice into a squib, but this can be pretty messy and you need a new set-up for each take.
So this is actually really interesting and not far from what @Snow said.
Before we get started, all the following information is coming from Joshua Meltzer, the master prop designer for Dexter.
Meltzer actually created his own blood formula for the show, consisting of maple syrup, food coloring, peppermint oil, and Dawn dish soap. Maple Syrup adds the thicker texture and cleans easier than corn syrup, peppermint oil repels the insects that are attracted to the syrup, and the Dawn soap makes the fake blood easier to clean out of clothes.
So when Dexter collects the blood samples, the blood is the maple syrup blend, but when we see the case of samples the dried blood is actually furniture dye.
Now, finally to your question.
In these scenes, Dexter uses a rigged knife, which is essentially a dull-blade knife that has a "blood pool" on one side (the knife below is the one used on set).
They have knives with blood pools on different sides to get cuts from varying angles.
Meltzer was actually showing off this knife to a reporter.
I hope this was helpful :)
