Actually, now that you ask, Blucher has the better range. The 12"/ 40 guns on the Formidable, London, Duncan, and King Edward VII class pre-Dreadnoughts had a range of 15,150 yards, at 13.5 degrees, which was the maximum elevations for those mounts. Blucher's 21cm (8.2") /45 guns had a range of 20,900 yards at 30 degrees, which was the maximum elevation of Blucher's turrets. Lets say we are talking about HMS King Edward VII as the ship sent to bottle up Blucher and sink her if possible. . Rate of fire for the British 12"/40 was 1.5 rounds per minute, giving a RN pre-Dreadnought a broadside of 6 rounds per minute. Blucher's main battery had a rate of fire of 4-6 rounds per minute, probably lower at higher angle of elevation so lets say 4 rpm, for a broadside of 32 rounds per minute.
Blucher had 2-2.8" deck armour and a 7.1 inch belt over critical parts. King Edward VII class pre-Dreadnoughts had a 9" main belt and 2" deck of mild steel, reduced to 1" in the centre of the hull. So Blucher had a better range, similar or better armour and much higher rate of fire. However, the British 12" gun fired a 850 lb shell, whereas the German 21cm gun fired a 237 lb shell.
I can find no armour penetration stats for the 21cm shell. The British 12" /40 could penetrate 12" of Krupp Side Armour at 4800 yards with AP shells or 9.7" of Krupp Armour at 3000 yards with Common Shells. How would this perform in the real world? HMS Good Hope, HMS Invincible and HMS King Edward were similarly armoured, the pre -dreadnought having two more inches of belt armour. Scharnhorst and Gneisenau sank Good Hope with their 21cm guns without much difficulty, although they may have caused the ship to be destroyed by uncontrollable fire reaching the magazines rather than by armour penetration. S&G hit Invincible and Inflexible lots, but did no damage, and were wrecked and then sunk by the British 12" shells.
Blucher would be unlikely to be able to use her full range, because fire control of the day was not up to the task. The Battle of Coronel began at 12,000 yards and closed to 6000 yards. The Battle of the Falkland Islands began at 16,000 yards but the deadliest shooting happened at 12,000 yards.
So how would Blucher vs. King Edward VII go? Probably not quite as well as the Falklands if Blucher was stationary. At those ranges both sides could use their 6" batteries, and King Edward VII had a pair of 9.2" secondaries on each broadside to complicate things.
The odds look a lot closer than I imagined.
www.navweaps.com
www.navweaps.com
en.wikipedia.org
en.wikipedia.org