That is a gel-filed external joint closure. Basically the two wires of the pairs from the two cables are joined with gel-crimps and then the gel-crimps are enclosed in a gel-filled enclosure. Double water-proofing!
If Openreach ever needed to do anything with their wiring, pre the NTE5, that joint would be opened.
As for opening the paint-encrusted BT52A (the BT stands for Block Terminal), gently scrape out the paint in the screw-head slot, insert a screwdriver that completely fits the screw-head slot and then gently tap the end of the screwdriver with a hammer. Be firm but gentle and you should be able to easily open it.