Hey! Glad to hear you're having fun :)
Your assumption is correct. The maximum accuracy bonus derived from the training activity for Darkness is +60, which does indeed only train it up to -60 actual stat (from -120). I'm considering a possibility of increasing it even further at some point, which will make Darkness a formidable party member, but for now (and to stay in line with her character) this is as far as it goes.