Using a font this heavy requires some expertise. Follow these rules:

Hiragana characters (e.g., あ, い, う) retain their distinctive curves despite the heavy weight. The challenge of a Black font is that delicate curves can become blobs. Kozuka succeeds here—the loop of 'あ' remains open and recognizable. Kanji with many strokes (e.g., 機, 鬱) are compressed but not illegible. However, at very small point sizes (below 14pt), multi-stroke kanji in this weight can become muddy.

Given its extreme weight, is not a "body text" font. Here are the ideal applications: