"HE JUST GOT in the door from Chicago a few minutes ago," Johnny Blackwell's wife tells a caller. Author of Johnny Blackwell's Poor Man's Catalog ("the world's most unusual do-it-yourself catalog," proclaims the cover), Blackwell is a computer specialist for the Defense Department who's parlayed a self-publishing venture into a sleeper success (47,000 copies, in trade paperback, have come off the press) for St. Martin's. Nineteen cities in 21 days has left the energetic Blackwell only slightly winded; fog kept him out of Seattle but he's been asked back for repeat performances in five of the cities he stopped in. And, he boasts, an appearance on "Good Morning, America" ("we had a hotel, a limousine, a producer, a writer and three rehearsals, all for three minutes") got him extra bookings in St. Louis and Nashville. Blackwell speaks in the royal "we"--there's only one of him out on tour--but he's just folks, really, and his schtick is demonstrating some of the easier, more portable do-it-yourself projects for which his catalog provides designs. What he calls a "humane" mousetrap, made out of a mayonnaise jar, for example, or a fishing lure from a toothbrush.

Blackwell says he jammed the switchboards in Miami when he did a phone-in show. "I stayed an extra hour and a half to help answer the phones and missed my plane to Atlanta." He also comments on the interest shown in his book not so much by the talk-show hosts and hostesses who were chatting him up but by the TV crews, "the guys with the cameras and sound equipment," who crowded around him, fascinated, after almost every program. Now, with his accumulated annual leave almost exhausted, Blackwell's back in Highland, Maryland, eager to get going on more money- saving ideas, to be collected in--what else but?--The Poor Woman's Catalog.