You could set the comment input format below the box to Full HTML, then do this: <span style="color:transparent">This is a spoiler</span>
Result (select text below to display): This is a spoiler
More information about formatting options
You could set the comment input format below the box to Full HTML, then do this:
<span style="color:transparent">This is a spoiler</span>
Result (select text below to display):
This is a spoiler