"Blue rinse brigade" is a somewhat pejorative

Pejorative

Pejoratives , including name slurs, are words or grammatical forms that connote negativity and express contempt or distaste. A term can be regarded as pejorative in some social groups but not in others, e.g., hacker is a term used for computer criminals as well as quick and clever computer experts...

 term used, particularly in the United Kingdom

United Kingdom

The United Kingdom of Great Britain and Northern IrelandIn the United Kingdom and Dependencies, other languages have been officially recognised as legitimate autochthonous languages under the European Charter for Regional or Minority Languages...

, to describe elderly middle-class ladies usually of a conservative socio-political persuasion. This group is usually characterised as forming the backbone of local branches of the Conservative Party

Conservative Party (UK)

The Conservative Party, formally the Conservative and Unionist Party, is a centre-right political party in the United Kingdom that adheres to the philosophies of conservatism and British unionism. It is the largest political party in the UK, and is currently the largest single party in the House...

.

The term arises from the blue rinse

Blue rinse

A blue rinse is a dilute hair dye used to reduce the yellowed appearance of greying hair on older women. The ability to see blue decreases with age due to the development of cataracts , so an older woman perceives her uncolored hair to have a yellow-tinge, and the blue rinse brings the color back...

 dye applied by hairdressers to disguise the often yellow tones of greying hair, together with the pejorative sense of "brigade"

Brigade (pejorative)

The word brigade, originally used to describe a military unit, can also be used as a pejorative collective noun to describe an informal group of like-minded individuals with views with which the speaker disagrees...

. There is also an allusion to the political symbolism of blue in the United Kingdom.