A tenacious defense limited Spain to just 19 of 44 shooting inside the arc and two of 22 beyond it.
Blümel's boys also won the rebounding battle, 42-29.
"For me it was the first German victory against Spain that I've experienced" Blümel said.
"It was all the more beautiful that we have succeeded in doing this in Spain."
The U18 European Championship for Men tips off in France on July 23, and both Germany and Spain are in Group B with Ukraine and Croatia.
Fabulous France Lord It Over Latvia
Latvia were handed a basketball lesson by a travelling French team ahead of the U18 European Championship for Men in Metz.
If their performances against Latvia in Riga are any indication, hosts France will threaten for a place on the podium.
After winning a couple of nail-biters, 58-57 and 68-65, the French stormed to a 90-66 win in the third and final friendly between the sides.