I cannot dance upon my toes,
blanch a troupe
To make the entire dance company turn pale with shock. Dickinson imagines her mental performance so brilliant it would terrify professional dancers.
lay a Prima mad
Drive the prima ballerina insane with jealousy. Note the violent verb—'lay' as in 'lay low' or strike down.
Eider balls
Eiderdown is soft duck feathers used in luxury bedding. She's describing tutus as puffy balls of expensive material—both mocking and precise.
out of sight in sound
Reverses the expected phrase 'out of sight and sound.' The applause continues even after the dancer disappears—sound outlasts sight.
placard boast me
No poster advertises her. But 'placard' as a verb is odd—she turns the advertising poster into an action it can't perform.