One thing you failed to tell us, was that this line was showing only in Live preview-mode, which I discovered now that I finally got my hands on the 1.1 version. And it actually turns out to be a border line on the surrounding element. That means, targeting that box will most likely address other elements has well.
With that being said, the following does target the blue line:
div.internal-embed.inline-embed.is-loaded {
border-left-style: none;
}
Now, in my simple tests this didn’t affect embedded image, but it did affect embedded notes, like in ![[SomeNote]]. Some more searches indicated that using the :has(.excalidraw-svg) would solve our issues, but sadly as of now it seems like Chromium is on version 100, and it need to be on version 105 for that to be supported, see here.
I then looked in Developers Tool again, and saw that the excalidraw image had another attribute in the div, which might narrow it down a little, and it did, so maybe this is your best bet until the underlying Chromium engine is updated to 105:
div.internal-embed.inline-embed.is-loaded[ready] {
border-left-style: none;
}