I take blood...
We advise not to do strenuous activity or lift heavy objects with the arm used for venepucture because...
it increases the blood pressure in that arm, this can result in the venepucture site bleeding again or massive bruising around the area, if the vein leaks....
I have never had issues traing after a blood test, probably is more pertanant to people with clotting issues, elderly or those with dodgy healing, or high blood pressure. Its a blanket statement, to cover any issues that may arise.