Hi,
Reading your post I seem to think you may have two separate issues with your injector.
Firstly, the black glaze is likely to have been caused by a leaking/split/distorted seal ring which you have possibly rectified by fitting new seals. One question, did you also replace the thin O ring in the head where the injector fits or just the two thicker injector seals?
I would assume the blackening was caused by exhaust gas forcing its way past the seals?
Secondly, before you condemn your injector it's likely that the top blue bakelite part of the injector is getting wet with petrol because of a leak where the short rubber hose and collar are fitted to the inlet stub. Remember that the fuel pressure here is very high and the slightest weakness in that area will cause a visible leak and if left unattended, an engine fire!
Before spending a lot of money on new injectors have a look at this kit on eBay, it's got everything you need to overhaul all 4 injectors to ensure that they are properly petrol-tight.
https://www.ebay.co.uk/itm/Volvo-P18...0AAOSwEYpb4KES
The kit doesn't include the thin O ring I mentioned but they are available from Brookhouse separately.
Hope this helps, Paul.