Personally it’s an intangible thing by nature.
However I would say solid grasp implies you’re able to pull off those techniques against people of those same belt level with comparative athleticism and size.
I would say that that you should be held to an individual and community wide standard. What I mean is that individually you may have the attributes combined with the skill to beat a blue/purple/brown/black, however that doesn’t make you one.
Blue’s should be able to explain (What is the goal, where are my limbs, where is my weight, etc) and be able to apply the basic positions (closed/half guard, side, back, mount, kneeride) against other blue belts of comparable attributes.
Ability to explain the basic submissions (armbar, triangle, kimura, americana, omoplata, guillotine, cross choke, rnc, arm triangle, footlock), basic sweeps (hipbump, scissor, hookflip)/guard passes breaks (double under, knee slice, single under, torrendo), escapes from mount/side/back.
Demonstrate and apply 2 basic takedowns of their choice, as well as the sprawl.
Demonstrate headlock escapes, standing guillotine defense, posture control from guard, standing in base, getting to the clinch while blocking punches.
You should be able to do gi and nogi. It sounds like a lot, but welcome to the world of grappling you gotta know shit.
I do agree with JNP assessment that they should have an area of strength. I would apply that later down the line to include a preferred guard, and a working knowledge of all guards but not the ability to use them. Only the ability to pass and defend.