How blue?
Could it be loom oil? that's not quite black??
Also they could have written on the backing the name of the customer, I've seen that happen numerous times, Ok you may say why didn't the full name show BUT it could be where they had rested the nib just before they were going to start writing on just ended.
May need to lift carpet.
One for the carpet retailer, if you try it it's your problem if it spreads or goes away and then returns.
Shaun