Is it possible to extract RNA from frozen blood?

It is NOT recommended to extract RNA from frozen whole blood.

To recover RNA from frozen blood samples, you can try to mix QIAzol at 10:1 ratio (QIAzol:blood) to frozen blood so that the blood is in contact with the QIAzol as it thaws. Once thawed and mixed with QIAzol, wait and allow it to come to room temperature and continue with the RNeasy Lipid Tissue protocol. Please do not heat the mixture, just allow it to come naturally to room temperature. Please be aware that these are just suggestions and the results are not guaranteed.