Except 120 miles north, around L.A., in the 70's and early 80's brown BF & SF Fenders, several types of Marshalls & Vox AC30's & AC15's were very popular among gigging players. Many ended up modded before "vintage" and "collector's" item were even known words in the guitar world. There were tons of classic rockers and country rockers, most using this type of gear. A decent percentage of the owner/players also had enough background in electronics to understand what needed to be serviced and when. And many of us that used SF amps from the 70's modified them for adjustable bias, often making changes in the phase inverter that matched the BF amps. So were were "blackfacing": amps and just calling it "modifying". I first saw the term in early issues of Vintage Guitar Magazine, which began publication in '86. I don't remember who used it first, but as I recall Gerald Weber really popularized it in his articles, many of which later made up his first book - published in '94. I hate to give him too much credit - but he seems to be the one that really made the term a common part of the guitar lexicon.