Have not tried to rust blue Rugers but have blued many with conventional hot blue.
The older ones re particularly hard to get a deep black color.Normally blue at 280f and with the older cast receivers typically 300-310. It's a fine line. Not hot enough and you will get a dull black/gray too hot you will get a rust red film that can only be buffed off. Essentially start over.
Rugers can be a PIA.

Lefty C