I am a 20 year old UK citizen (where I have lived all my life) and my mother had a French passport when she was a child but not since.
Is it possible for me to apply for French citizenship on blood grounds?
(Any naturalisation etc isn't possible b/c language/residence)
Assuming your mother was born in France, was a French national and did not renounce her nationality, my understanding is that you are already a citizen and don't need to "apply" for citizenship. To prove it to the authorities that would grant you a passport though, you might have to do some work.
If your mother had the foresight of having your birth certificate transcribed to a French Livret de famille when you were born, then the Service central d'état-civil should already be able to deliver a birth certificate to you. You can order one online here: https://pastel.diplomatie.gouv.fr/dali/index2.html
Otherwise, you would need to get a certificate of French nationality from the "Service de la nationalité des Français nés et établis hors de France" as described here https://www.service-public.fr/particuliers/vosdroits/F1051
[Update] You've clarified in comments that your mother was not born in France.
According to what I could read about the certificate of French nationality, obtaining it can be a long process lasting more than one year. In order to constitute an application, you need to (a) prove your filiation to your French ancestor, and then (b) to prove that they had French nationality.
In order to do (a) you need to provide birth certificates and marriage certificates of everyone up to the ancestor whose citizenship you can prove.
So the first question you should answer is, can you prove your mother's French nationality directly, ie do you have documents, such as a former passport of hers, that could be used to prove her nationality? I assume you don't, which means that since she was born out of France, you'll need to go back up to your grandmother.
I'm also going to assume you don't have any documents pertaining to your grandmother. To prove that she is or was French, in addition to her birth certificate, you would need her own parent's birth certificates and marriage certificate (they would be your French great-grandparents).
For documents less than 100 years old, birth certificates are issued by the Mairie of the city of birth and marriage certificates are issued by the Mairie of the city in which the marriage was celebrated. For documents more than 100 years old, these documents are issued by the direction departmental des archives.
This might sound like a lot of work. Also when going back so far up, there aren't any written hard-and-fast rules. Maybe you'll want the advice of counsel with experience in this domain. Good luck!
Most of the information here is repurposed from this document published by the French consulate in Argentina: http://www.embafrancia-argentina.org/IMG/pdf/CNF.pdf?3513/b39e2934f8d6bcccea534569a5241a35407fc931
