A perfect use for the eBay Bulk Edit And Replace (BEAR) tool.
I know nothing about accessing it if using Seller Hub, but if using Selling Manager, or My eBay you go to your active listings, select the ones you want to operate on, choose edit, and then the bulk editor pops up and you proceed from there.
How you proceed depends on the structure of your listings. If just adding the blurb to the end of them works for you then you choose that option. If you need to slide the new blurb in the middle of the listing then you need to use the bulk editor in search/replace mode and figure out something in the listings that is a consistent "hook" you can use as a search term. You search for that constant thing in the listings, and replace it with that thing + the new blurb.
Be aware that you have to search and replace in HTML unless the target and BLURB are simple plain text.
An example of the simple text case would be if you had the phrase "Ask me about anything else via Contact Seller" in all the listings and wanted to insert BLURB after it. You would search for "Ask me about anything else via Contact Seller" (no quotes), and replace it with "Ask me about anything else via Contact Seller" BLURB (no quotes)
Shipscript has a good BEAR tutorial here:
