None of the barrels Ive seen finished by Johnsons are plum black. I have no idea what that is actually, other than bluing/blacking that has aged without being oiled properly....or in some cases with hot bluing...that part hasnt been in the solution long enough...or it was in too long...or the solution is too old, etc.

There is a guy in Wisconsin that says he can replicate best English type blacking...Ive never seen his work, Ill post a link just for reference.
http://www.rgsrestorations.com