No he isn't. It's impossible for several reasons.
1. Johnny had been calling Cassie's phone right before Sonya picked it up. How could he do this from another realm exactly? And why would he when, if he were Erron Black, he'd know exactly where she was given that he was the one who captured her and Jacqui?
2. Sonya talked about letting Johnny be the one to deal with Cassie after she left the base, and later gave the order for Johnny to come along on their mission to Outworld. The way characters talk about him, he's obviously still in Earthrealm, so how could he maintain a double life in Outworld? I don't think any charade like that would last for long, especially when it's clear that Sonya and the Special Forces wouldn't be involved.
3. Johnny isn't blonde. Erron is. (maybe, depending on the colourist)
This dumb theory seems to have come about solely because Johnny calling Cassie's phone and a photo of Erron on her phone caused people to put some sort of association between them.