Fabric roller blind, which comprises: - a support frame provided with at least two vertical uprights and intended to be fixed in an opening of a portal, of a window or of a door; said two vertical uprights extended parallel to each other and side-by-side fixed to said portal along respective sides of said opening; - a take-up roller rotatably supported by said support frame, between said two vertical uprights, along a substantially horizontal rotation axis, and intended to be positioned in the upper part of said opening; - a fabric extended between an upper edge thereof fixed to said take-up roller and a lower edge thereof, and susceptible of sliding between said vertical uprights; - at least one return spring mechanically associated with said take-up roller and adapted to exert an elastic torque on said take-up roller; said take-up roller being susceptible of being rotated around the rotation axis in order to move said fabric between a retracted position, in which said fabric is wound around said take-up roller and an extended position, in which said fabric is at least partially unwound from said take-up roller; said return spring being elastically opposed to the unwinding of said fabric from said retracted position to said extended position and facilitating the re-winding of said fabric from said extended position to said retracted position; - adjusting means for adjusting the tension of said return spring, mechanically connected to said take-up roller in order to vary the tensioning of said return spring; said blind being characterized in that said adjusting means comprise: - an actuator member, rotatably mounted inside at least one of said uprights of said support frame, accessible by means of a hole obtained on the same upright in a position facing the opposite upright, from within said opening and susceptible of being rotated into different angular positions; mechanical engagement means being provided for mechanically engaging said actuator member with said upright in said angular positions; - transmission means for transmitting the rotation torque of said actuator member to said take-up roller, consequently varying the tensioning of said return spring. - a support frame provided with at least two vertical uprights and intended to be fixed in an opening of a portal, of a window or of a door; said two vertical uprights extended parallel to each other and side-by-side fixed to said portal along respective sides of said opening; - a take-up roller rotatably supported by said support frame, between said two vertical uprights, along a substantially horizontal rotation axis, and intended to be positioned in the upper part of said opening; - a fabric extended between an upper edge thereof fixed to said take-up roller and a lower edge thereof, and susceptible of sliding between said vertical uprights; - at least one return spring mechanically associated with said take-up roller and adapted to exert an elastic torque on said take-up roller; - adjusting means for adjusting the tension of said return spring, mechanically connected to said take-up roller in order to vary the tensioning of said return spring; - an actuator member, rotatably mounted inside at least one of said uprights of said support frame, accessible by means of a hole obtained on the same upright in a position facing the opposite upright, from within said opening and susceptible of being rotated into different angular positions; mechanical engagement means being provided for mechanically engaging said actuator member with said upright in said angular positions; - transmission means for transmitting the rotation torque of said actuator member to said take-up roller, consequently varying the tensioning of said return spring.