Most manufacturers assemble the barrel, handguard cap and front sight base and then park it. What you see is particles of bead blast media that was in nooks and crannies that lifts out when it is dipped. The rust looking stuff should come off when it is oiled. I use an M16 toothbrush to gently scrub it off.
What brand of barrel is it? I know some of the LMT barrels as well as all of the Colt barrels are parkerized before assembly, but lots of builders don't.
The oil coating should remove the marks on the dry finish.