on the blob

Apparently a reference to the blobbing (i.e. dripping) of menstrual blood, likely following the model of on the rag.

on the blob

1. (Britain, slang, vulgar) Menstruating; on one's period.
   • 1993 April 19, Sean McAfee, “The Great a.t. IRC Party, part III”, in alt.tasteless‎ (Usenet), message-ID <1993Apr19.224852.14466@mtu.edu>:

     <julian> I have no problems fucking wimin on the blob. But some of them do

   • 1997 July 3, Nicholas Lezard, “Paperbacks”, in The Guardian, page A14:

     Sweet's heroines are – well, unskinny; they eat lard, drink, smoke, ponce off the state, have huge dumps and go on the blob at inconvenient times.

   • 2012, Damon Beesley, Iain Morris, The Inbetweeners: The Complete Scripts with Added Extras, Random House, page 285:

     Katie Can you stop staring at my tits please? […]
     Jay She must be on the blob.

   • 2015 February 13, Caitlin Moran, “Celebrity Watch”, in The Times:

     Personally CW can think of no tampon in the world it would trust while wearing a white dress, on the blob, at a glamorous media event.

• on the rag