EntertainmentTVAction ShowsMarvel TV ShowsAgatha All AlongAgatha All Along fans have a new theory about Billy that shakes up what those “black heart” references could meanWhen you purchase through links on our site, we may earn an affiliate commission.Here’s how it works. EntertainmentTVAction ShowsMarvel TV ShowsAgatha All AlongAgatha All Along fans have a new theory about Billy that shakes up what those “black heart” references could meanWhen you purchase through links on our site, we may earn an affiliate commission.Here’s how it works. When you purchase through links on our site, we may earn an affiliate commission.Here’s how it works. (Image credit: Marvel Studios) Thanks to a line in Agatha All Along’s opening episode, we’d all assumed that the ‘black heart’ that marked the final name in Lilia’s list of potential coven candidates had been Rio. Now, though, it seems like that could’ve been a misdirect – and fans think …