A simple cable with just wires and no electronics inside, as the one your photo shows, will simply forward any voltage. It won't change it.
The label on the cable specifies the maximum voltage it was made for. You can use it for any lower voltage as well. The
~ says that the voltage is alternating, in contrast to
= where the voltage is constant (aka. direct).
I'm not sure how many different Waldorf Blofeld models exist, but one looks like this:
If your keyboard has this connector labelled AC In (alternating current in), then your cable will fit into the wall on one side and into the keyboard on the other side.
No serious electronics engineer would construct a 12 V= plug with that kind of connector, simply because everybody expects to put 110 V~ or 230 V~ into it.
A typical low voltage DC connector looks like this:
So there's no way you could accidentally mix a high voltage AC cable with a low voltage DC plug.
If your keyboard has a low voltage DC connector, then you need a power supply that converts 110 or 230 V~ to 12 V= like this (should have been shipped):
As you can see, it has the high voltage connector on one side and the low voltage connector on the other side.