Luminol is used to detect the presence of blood, it doesn't find all sources of DNA.
Normally the presence of blood is detected at a crime scene by close visual examination and then presumptive testing to make sure it really is blood is done before samples/swabs are collected for lab analysis. The scene is photographed extensively with evidence markers in situ so that there is documentation of exactly where samples came from.
Typically Luminol would be used to find blood that was not visible due to being in such small amounts that it can't be seen by the naked eye, or having already been subjected to cleanup attempts. I don't think that the crime scene in Delphi would likely have a miniscule amount of blood based on court documents that we know about so far; nor do I think it's really possible for clean up to have occurred based on the outdoor nature of the crime scene. I do think luminol could possibly have been used to detect/map a route of exit from the crime scene, document drag marks, something along those lines.
For example, the markings on the tree that appear to have been made in blood. You don't need to luminol those because visually, you see that blood is there. Swabs can be taken from the markings that are visible.
@Megnut is absolutely correct that not every swab or sample is tested. Some end up being unsuitable and others are redundant. The detectives and the lab make these decisions in concert with each other based on known information at the time. It's an imperfect process.
All of this is IMO.