Another Thing To Check

I have just discovered a leak in my LPG line. The gas bottle only lasted 12 days with little gas stove use so I figured there was a problem!
On filling the bottle it was noticed that the telltale started spitting straight away, prompting the refiller to comment that perhaps the bottle hadn't be filled up on the last fill.
Just to be sure that was the problem, I ran the soapy water test on all the junctions and fittings I could access.
Surprised to find a joint on the isolating tap was bubbling.

The joint hadn't been touched since the van was new (6 years). I had taken the stove out and the fridge for repairs so expected a leak there. Didn't smell anything in the van over the past few days while the bottle was emptying itself.

Too off the junction and cleaned it up and no bubbles.

Question - inside the joint I took off, there was a black residue on the threads inside the locking nut (the bit that goes over the olive dome). Is there anything to worry about having "dirt" inside the pipe? What is it and where did it come from?

Thanks guys

bill