however, how would a blind person know where to click???

They put instructions for the blind in some code that is invisible to the sighted user because it is defined (in CSS) to be the same color as its background. Something like this:

<p style="background-color:white; color:white">Use the following link for an <a href="http://big-web-site.com/cgi-bin/audio-captcha" style="background-color:white; color:white">audio version</a> of this security feature for the blind or visually impaired.</p>

I could show you how this example will look except that embedded "style" attributes are ignored by PM.

While the ordinary user will never see this text or link unless they highlight it, Lynx users and users of audio-based browsers will see the text just fine, as these browsers ignore the color properties of CSS.

Hope this helps.