Findings and brief analysis
Googling about, here's what I've found so far.
- 過去の否定形 -- 35,400 hits
Of the first page of 10 hits, 6 were about French grammar, one about Mongolian, one about Korean, one about English, and one about Japanese.
- 否定の過去形 -- 27,200 hits
First 10 hits: 5 about Japanese, two about Ryūkyūan (Okinawan and Miyako), two about Korean, and one about English.
- 過去否定形 -- 36,200 hits
First 10 hits: all 10 about Japanese grammar, but all 10 targeting a Chinese-reading audience.
- 過去否定形 + は (the "は" added to filter for just Japanese sites) -- 8,870 hits
First 10 hits: 5 about Japanese grammar, one about Korean, one about Finnish, one about Russian, one about Thai, and the generic page about negative verbs at https://ja.wikipedia.org/wiki/否定動詞.
- 否定過去形 -- 10,100 hits
First 10 hits: 9 about Japanese (3 clones of a Japanese question about describing Japanese grammar in English, 3 pages explaining Japanese grammar to Chinese readers, a Japanese Wikibooks page for non-native learners of Japanese, 1 in Japanese asking about Japanese grammar, and 1 in Japanese about Kansai-ben), and 1 about English.
- 否定過去形 + は -- 9,550 hits
First 10 hits: 8 about Japanese (4 clones of the Japanese question about describing Japanese grammar in English, 3 about Japanese grammar, 1 about Kansai-ben), 1 about English, and 1 about French.
Given the pattern of conjugational endings in Japanese, negation comes first, and then the past tense suffix happens on the end. So the more natural Japanese-y way of thinking about it, and ordering the terms, would be 否定【ひてい】 and then 過去【かこ】, which might be why the variants with this same order seem to be used more specifically for talking about Japanese grammar in Japanese.
There doesn't seem to be a set phrase for this, with some variation. However, the most common for talking about Japanese grammar appears to be 否定の過去形 or 否定過去形.