How can 盼 refer to black and white?
This is not a Waley phenomenon. The majority of English translators of Shih-ching (in the absence of 素以為絢兮) follow strictly the preachings of Shuowen Jiezi when interpreting 盼 in 美目盼兮, to varying extents of literality.





Translation





Legge 1871:95
How lovely her eyes, with the black and white so well-defined!


Legge 1876:104
With dark and lucid eyes, showed face beyond compare.


Jennings 1891:83
Beautiful eyes, so dark, so bright!


Waley 1937:80
The lovely eyes so black and white.


Karlgren 1950:38
Her beautiful eyes are well-defined black and white.


Pound 1954:28
Clear white, gainst black iris.




Legge and Waley also translated III.8 of The Analects in the same manner. Some other translators of The Analects choose to interpret the quote (in the presence of 素以為絢兮) more liberally, but still do not use the more common verbal meaning 'to look' except Lau 1979. Note even in Lau's interpretation the much derived meaning 'to hope for' is not adopted.




Translation





Lyall 1909
Her lovely eyes, So clear and bright


Soothill 1910:191
Her bewitching eyes shew their colours so clear


Lau 1979
Her beautiful eyes glancing




My take on this matter is that 素以為絢兮 may be a Confucian (in the sense that the dialogue between Confucius and Tzu-hsia that follows it centres on not the quoted poem, but 絢 as a metaphor for 禮) annotation of the preceding verse 美目盼兮; or a piece of yishi (逸詩, lit. lost poem). If the former be the case, the contrast of colour between 素 and 絢 may also help justify interpreting 盼 as 'black and white well defined'. See more of  素以為絢 here. Another weaker (and potentially flawed) justification is that in order for the couplet 巧笑倩兮，美目盼兮 to have matching parts of speech, 盼 should be adjectival.

This article, however, unearths several annotations of the phrase of concern that provide a different explanation:


通志堂本《经典释文·毛诗音义》作“盻”，并注“敷苋反，白黑分也。徐又敷谏反，《韩诗》云‘黑色也’，《字林》云‘美目也’，匹间反，又匹苋反。”


阮刻《论语注疏》在原文下有小字注“马曰：‘倩，笑貌；盼，动目貌。”

Apparently, not every source listed above agrees with the Shuowen Jiezi school. Especially striking is the interpretation of Ma Rong, an Eastern Han scholar and a contemporary of Xu Shen. But this was not published until the Qing Dynasty in the Ruan ke annotation, which may weaken the credibility of the quote.