From the datasheet

Control bits (see Fig.4)
The control bits C0 to C6 have the following meaning:
C0 = 0 static mode, i.e. continuous display of digits 1 and 2
C0 = 1 dynamic mode, i.e. alternating display of digit 1 + 3 and 2 + 4
C1 = 0/1 digits 1 + 3 are blanked/not blanked
C2 = 0/1 digits 2 + 4 are blanked/not blanked
C3 = 1 all segment outputs are switched-on for segment test(1)
C4 = 1 adds 3 mA to segment output current
C5 = 1 adds 6 mA to segment output current
C6 = 1 adds 12 mA to segment output current


Since the dynamic mode supports alternating display of digit 1+3 and 2+4, thats why balnking is also supported on 1+3 and 2+4.

That is just a guess and your question about balnking of these 1+3 and 2+4 is still un-answered. May be Philips people can answer this ?