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Usually, there is little to no blood if a cadaver is dissected/dismembered some hours after death – IIRC, it's around 6 hours. In some few cases there can be a release of post-mortem blood from injuries sustained before or even after death, but I believe only rarely does this occur.
Also, blood after death has different properties from that released by a living body. Simplistically, it is less liquid, more coagulated. I imagine a forensics serologist could determine if blood found at a crime scene was released by a living person, or was from one who was already deceased eg. were it to be found at all on, for example, clothing worn or tools used by a murder suspect.
What I'm trying to convey as delicately as possible is that if BM strangled or drowned SM at a particular time, and some hours later prepared her body for removal in, for example, a number of coolers to be concealed perhaps in several locations, there may have been very little blood, or even no blood, needing to be cleaned up at the site used for body preparation.
Doubtless some perhaps even minuscule body tissue would have been missed in any clean-up by BM, yet retrieved by LE and its DNA identified.
All the above is solely my inexpert postulations.