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 :)