Commerce raiders sink merchant ships. How many German commerce raiders were sunk by British Battle Cruisers? The merchant cruisers that Germany put to sea, including an old sailing ship did just fine on the sea lanes. You gotta find 'em to sink them. How long is the RN going to keep BCs at sea chasing around after a big German commerce raider. One needs to read the history of German World War I commerce raiding to judge the Blucher. If she goes down to 22 knots, who cares? Nobody. You don't need 22 knots to catch merchant ships. When and if a British Battle Cruiser finds you, you are sunk, whether raiding in the North Atlantic or chasing the tail of your own BC on the Dogger Bank.
I'de assume Blucher at sea would sink 40 merchant ships before anyone laid a glove on her. In fact, the more British BCs that sail around in circles searching for a needle in a haystack, the better. Gives the German High Seas Fleet BC force a clearer shot at bombarding the UK coast and drawing an inferior RN BC force to certain distruction. The war at sea had to be fought in many places at once. RN BCs out in the Atlantic for months on end? That would make the High Seas Fleet all the happier. Mission accomplished Blucher. Use your ships where they can pay off, Blucher in the North Sea was a zero, a waste of men and guns. I doubt any merchant ship she found in the Atlantic would feel she was a zero.
Bob B.
That's assuming that two of the RNs fast and modern light cruisers haven't tracked such a ship down and sent it to the bottom or at least inflicted so much damage that such a warship { Blucher] would have been so much scrap metal.