If it is frozen sample, Trizol is best. If it is fresh you have more options. Depending on what you plan to do with the RNA, there are lots of things you will need to do downstream after isolating RNA from whole blood with Trizol to find meaningful results. There is also a kit from Zymo Research that allows you to directly apply Trizol treated samples directly for column purification. It all depends on how many blood samples you will need to process in parallel. Good luck!