Did the name "白俄罗斯" exist in Chinese before 1991?
the internet archive has the book 瀛環志畧, in volume 4, "白峩" was used to translate belarus:

https://archive.org/stream/02087085.cn#page/n43/mode/1up



this book was published in 1849.

the 峩 (u+5ce9) was a variant of 峨 (u+5ce8), both of them share the same pronunciation with the character 俄 (u+4fc4), that is used in modern days translation.

then, the internet archive has another book 海國圖志, in volume 54, "白峩羅斯" was used in lieu of belarus:

https://archive.org/stream/02087117.cn#page/n50/mode/1up



this book was published in 1843, maybe the first geographic / atlas type of book in imperial china.

btw, minsk was translated as "閔士其" :)

have fun :)