Is a true adjective a word that cannot be used as a verb? A "red" apple; a "blue" sky. Are true adjectives words that cannot be used as verbs?

In my younger days, the weekly wash was "blued;" i.e., blue powder was put into the water to make the laundry look whiter when it emerged. I think some modern washing powders use a similar technique.
Also, when all of your money had been spent, particularly on riotous living, one was said to have "blued" it. Not sure of the etymology of that one, though. Perhaps a corruption of "blown", as in "blown away"

And the verbal noun is the gerund, the verbal adjective the gerundive, I believe. (I gleaned that from either Fowler or Partridge, almost certainly, says he, religiously acknowledging his sources