I don't get the reference to deep blue, but perhaps I'm translating it wrong?

From WWWJDIC (https://www.edrdg.org/jmdictdb/cgi-bin/entr.py?svc=jmdict&sid=&q=1604080):

真っ青 [ichi1,news2,nf28] ； まっ青； 真青 [io]
【 まっさお [ichi1,news2,nf28] 】
1. [adj-na,n]
▶ deep blue
▶ bright blue
Cross references:
⇐ see: 2771600 真青【まさお】 1. deep blue; ghastly pale
2. [adj-na,n]
▶ ghastly pale
▶ pallid
▶ white as a sheet