Athletes already do this - they collect their own blood and store it and then transfuse themselves to supranormal levels prior to their sporting event. Lance Armstrong did this.

Lab grown blood would make this a bit more convenient, albeit at astronomical cost.

I assumed the reason to use their own blood was to avoid detection. Presumably synthetic blood is weird enough that it becomes trivial to detect its presence that would make it a no-go for cheating.

“Synthetic” in this case doesn’t mean that it’s made using an alternative chemical process - it just means they’re not made in bone marrow.

This process filters out stem cells from a blood donation and prevents them from becoming red blood cells long enough to multiply them significantly.

Yes. Some of the proteins in the cells would be different to the host cells, due to the exogenous stem cell source. This might be detectable with mass spectrometry (protein sequencing is otherwise difficult and laborious). There might also be different post translational modifications on the lab cells.

Nowadays they just take EPO

True. I imagine a scenario where the athlete is given supranormal blood without the extra step of collecting from themselves.

This is just me speaking “out loud”. Today, this would be quite expensive. I can see a future where the technology scales and the richer teams/individuals try this out.

The salient point is it's prohibited in sports where it gives you a boost, and there are already ways to do it today.

Lance Armstrong was busted for blood doping. The cutting edge is way beyond that.