Yes - blood must be kept at very specific temperatures and conditions if you want to preserve its usefulness in DNA sequencing - most of these are dried blood cards without strong controls which makes them not particularly useful for good DNA data.

Scientists have extracted usable DNA from from 7000 year old corpses. I imagine the technology exists for a decades old blood sample if it’s stored at least somewhat properly.

Yes that is very true - There is however quite a bit of difference between a full corpse that is mildly preserved and a dried fleck of blood - a big difference too between a stored fluid blood sample and these pinprick dried sample.

It's probably prohibitively expensive at the moment.