A search box would be perfect for big projects with a lot of custom blocks.
It would also just be a convenient feature that doesn't seem that implausible to implement, at least from my perspective.
In the top right of each palette of blocks there's a little magnifying glass, next to a + button. If you click that magnifying glass, you'll get to a search box that lets you filter all the blocks in the project, regardless of what category you have selected.
To add to what @cycomachead says, you can also use Control+F to trigger block search, or shift-click on the scripts pane (programming zone) to enter keyboard input mode, in which you can look for blocks and build scripts by typing.
Oh haha thank you for informing me I didn't know these things 