I quote from the key to Stylidium species related to Stylidium graminifolium. S. graminifolium: leaves strictly narrowly linear to 2.5 mm wide, glaucous (with a waxy bloom), edges irregularly serrate (saw-toothed), flowers white to pale pink. In siliceous sands and well drained soils. S. armeria: Leaves greater than 3 mm wide, serrated at the apex only, 20-40 mm long, dark green, scape (flower stalk) robust, more than 2 mm wide, glandular hairy. Flowers dark pink to deep magenta. Widespread.
I've just checked two papers relevant to Stylidium graminifolium and S. armeria: Raulings & Ladiges (2001) Australian Systematic Botany 14, 901-935, and Jackson & Wiltshire (2001) Australian Systematic Botany 14, 937-969. Both provided keys and descriptions for the two species (though Jackson & Wiltshire note they only examined Tasmanian herbarium specimens for S. graminifolium. It’s unclear how many specimens Raulings & Ladiges looked at, but they don’t cite any ACT specimens for S. graminifolium, and only one ACT specimen (from Mt Franklin) for S. armeria [all the other S. armeria specimens cited in their paper under the ACT are actually from NSW]. I’ve checked all the ACT specimens in ANH for both species against the keys and descriptions in the above two papers; the specimens include many from Black Mountain. Based on the herbarium specimens and relating it to my field knowledge of the genus on Black Mountain, I don’t believe S. armeria occurs on Black Mountain, Bruce Ridge or Mt Ainslie. The main obvious differences between the two species are the leaves: S. graminifolium leaves are linear, up to 3 mm wide, and the mid rib is clearly visible on the lower surface. S. armeria leaves are narrowly oblanceolate (ie broader in the upper half), usually 4 mm or more wide (rarely c. 3 mm) and the midrib is not obvious on the lower surface. These characters are difficult to tell from photos but are readily apparent in herbarium specimens (and fresh specimens).