BooNZ
Banned
Well that is what you get for trusting a computer search, it seems the only people with the exact text are the National Archives at Kew who claim not to have digitised it.
However there are other sources who mention the Order in Council...lots of them here is just one, that it is a bit of a long winded and full on historical examination.
The interesting thing about the March 11, 1915 declaration is that although it describes a blockade its specific legal justification is in the concept of reprisal. However it was also recognised as stated above that British blockade practice conformed with earlier American usage and practice. Anyway worth recalling that you asked if the British declared a blockade and " Reprisals Order in Council 11th March 1915. Measures to prevent commodities of any kind reaching or leaving Germany" is that declaration.
Yes - suffice to say the British were not comfortable describing their actions as a blockade. It was simply a case the Germans could not prevent it and the US chose to ignore it, which is consistent with the US blockade of the CSA. However, ignoring the existing law or rights of neutrals only works if the neutrals are either complicit or lack the ability to effectively protest.