The easiest way to enable or disable ad-blocking from your pi-hole using Alexa is to install fauxmo – with which you can create fake Belkin devices that are Alexa-enabled.

You then configure fauxmo with URLs like this:

# disable for 600 seconds
http://YOUR.PIHOLE.IP/admin/api.php?disable=600&auth=YOUR_AUTH_CODE
# enable
http://YOUR.PIHOLE.IP/admin/api.php?enable&auth=YOUR_AUTH_CODE
# get status
http://YOUR.PIHOLE.IP/admin/api.php?status&auth=YOUR_AUTH_CODE

Get YOUR_AUTH_CODE from the WEBPASSWORD= line in /etc/pihole/setupVars.conf

See example conf at GitHub