What is the difference between the red and black box identifiers on UK airport charts?
(UK AIP GEN 2.3)

Both are holding position signs. Both reflect the physical sign color the pilot would see, below is the red J2 and black J1 as seen from the cockpit (YouTube):


Red J2


Black J1

The red is for intermediate holding positions:


  Where it is considered necessary to locate taxi-holding positions other than at runway/taxiway intersections – for example at taxiway/taxiway intersections in order to protect a priority route – the holding position should be identified by a single sign located wherever practicable on the left side of the taxiway. The sign consists of a combination of the letter designating the taxiway and a number identifying the hold position e.g. A1, A2, B2 etc. in white on a red background.


(CAP 168 Licensing of Aerodromes)