> If I remember correctly, ad blocking software overlays the ad with blank HTML right?

Don't know why you're being downvoted, you're simply unsure about a (wrong) assumption and asking about it, which to me simply calls for a correction / explanation. Anyway, no, uBlock for example actually blocks http requests, based on huge community-maintained blacklists. Just try loading theverge.com :

- Without uBlock: 131 requests

- With uBlock: 37 requests