Oil released when I removed rear hub dust cap

As anyone who has read any of my previous posts will know, I’m not the greatest at knowing what’s going on with my 2005 Defender LWB.
Yesterday, out of curiosity, I prised off one of the rear hub dust caps and was rather surprised when a load of black oil poured out. A dust cap presumably shouldn’t be the final barrier against an oil leak so I presume i have a problem further inside the hub?
Any advice?
Thanks
Geoff

in some ways lubing the shaft splines is a good thing and used to be standard land rover practice later abandoned,but there is a seal in the stub axle and its worth checking breather first as if its blocked oil will be forced out through the seals