What does str typing mean when dna is not detected in blood?

It means that the amount of sample is not enough to be analyzed by electrophoresis after amplification. Or the blood you provided is not blood or not human blood.

STR is a segment of a chromosome in the human body that repeats itself with a core of 2 to 5 bases, which varies from person to person and is inherited.

As long as multiple STR loci are tested, the identification of an individual can be done, which in turn allows DNA testing to be used in practice. A drop of blood, a hair or even a dander extracted from the scene of a case can be found in the identification of the crime. That's why DNA is also known as "blood fingerprinting".

Expanded Information:

STR typing based on capillary electrophoresis is performed using a Multiplex PCR complex amplification system, which can simultaneously amplify 19 STR loci plus one sex-determining locus, Amelogenin. Each human cell line analyzed has its own unique DNA repetition pattern, so that the species of each new batch of cell lines can be confirmed by comparing it with the basal map.

Often DNA paternity testing is applied to family kinship identification cases related to recognizing children, such as the recognition of long-lost parents and children, which plays a crucial role in meeting the kinship needs of the families concerned.

Baidu Encyclopedia - Cellular STR Typing