A few online sources indicate Blood joined the Roundheads at some point during the Civil War. Blood probably used the same techniques on Cromwell he used on Charles II. Cromwell is quite flustered by the flirting while his two most loyal Roundheads try to save him from heartbreak.

In short: Blood being a manipulative git, Cromwell being flustered by Blood's flirting, Roundhead!Ben being skeptical of blood, Roundhead!Simon being protective of Cromwell.