You can indeed see who has upvoted a musing.io user's question using the steem blockchain. Let's say I upvote this question (which I have) posed by @verbose-jimmy on musing.io:
https://musing.io/q/verbose-jimmy/f3jece4sx?r=answer
I go to find the user @verbose-jimmy on steemd.com:
https://steemd.com/@verbose-jimmy
From the musing.io link anybody who upvotes the question by verbose-jimmy/f3jece4sx will be shown as:
(steemit username) upvote @verbose-jimmy/f3jece4sx.
Using the "find on page" option for the internet browser search for "verbose-jimmy/f3jece4sx". In this example what gets shown is:
holovision upvote @verbose-jimmy/f3jece4sx