Blackwell doesn't count because he hasn't left the club yet and somehow I think his record might deteriorate this season!
Kendall counts, but obviously it's easier to improve your position over two seasons than five.
I'll change the question slightly. Have we ever sacked a manager other than Ian Porterfield who throughout his managerial term improved our league position year on year?
Which managers have we actually sacked?
1952 Davison - voluntarily left to Manager Chesterfield!
1955 Freeman -died
1958 Mercer - left to manage Villa
1968 Harris - booted upstairs after relegation
1969 Rowley - sacked after one season in charge and a disappointing 9th finish in D2
1973 Harris - retired by mutual consent I believe
1975 Furphy - sacked in October with United rock bottom, but in his two previous seasons had seen us finish 13th and then 6th
1977 Sirrel - sacked in September with United next to bottom of D2 after seeing us relegated from D1 in 76
1981 Haslam - booted upstairs after seeing United finish in a worse position in every season he was in charge
1981 Peters - resigned after relegation to D4.
1986 Porterfield - see above
1988 McEwan - resigned as United struggled against relegation from D2, after finished 7th then 9th in his first two seasons
1995 Bassett - left "by mutual consent" as United struggled in the 2nd tier
1997 Kendall - see above
1998 Spackman - resigned after one too many bust ups with Charles Green
1998 Thompson - not given the job in close seasion 98
1999 Bruce - resigned to manage Huddersfield!
1999 Heath - resigned after the infamous Vale game
2007 Warnock - sacked after relegation from PL
2008 Robson - sacked after the shoes off protest
So, in answer your questions, you could argue that Furphy was the only other example of a manager being sacked after improving our position in each season he had been in charge. But Furphy had only had two seasons and we were rock bottom of D1 when he went. Porterfield, on the other hand, had taken us from D4 to serious D2 promotion challengers (for the first time in 15 years) in 5 years. I agree his sacking was monumentally unfair.