How many blacks were living in England in 1348? England conducted no maritime trade with any sub-Saharan African nations and would not do so for at least another century, so any blacks who wound up in England would have had to come from Muslim Spain, Morocco, the Barbary Coast, the Mameluke Sultanate, or the Ottoman Empire.
The legends tell of a Moorish knight of the Round Table, Morien; to wind up in the legends, they were not unknown or all despised in England.