A blank verse with a feminine ending is, not the standard ten-syllabic metre, instead, one more than the ten, ending with an additional unstressed syllable, making it a metre of eleven syllables. The eleven syllables are five pairs of unstressed and stressed syllables, like five heart beats of dee-Dum, where one unstressed syllable is followed by a stressed syllable, until it is a metrical feet of five beats and ten syllables and then an additional unstressed syllable.
Like in this line from Shakespeare's A Midsummer Night's Dream:
When thou hast stolen away from fairy land, (II.i.434) When thou | hast sto | len a | way from | fai ry | land
if the verse ends with one less syllable — a stressed syllable less than the usual five — making it a metre of nine syllables.
Here's an example from Shakespeare's The Comedy of Errors:
Merchant of Syracuse, plead no more; (I.i.iii) Merchant | of Sy | racuse| plead no| more