The server can ask PuTTY to display text that blinks on and off. This is very distracting, so PuTTY allows you to turn blinking text off completely.
When blinking text is disabled and the server attempts to make some text blink, PuTTY will instead display the text with a bolded background colour.
Blinking text can be turned on and off by control sequences sent by the server. This configuration option controls the default state, which will be restored when you reset the terminal (see section 3.1.3.6). However, if you modify this option in mid-session using ‘Change Settings’, it will take effect immediately.