Astus makes the important point that "Zen" is not a single unified entity. It's a family of lineages with often unique styles and methods passed down in teaching lines. I would not say there is disagreement, though, on the general course and goal of practice even when "house styles" - practice methods and emphasis - differ. Again, a good teacher should be able to apply appropriate methods to help you deal with whatever obstacles arise, e.g. blankness/dullness in meditation. The calmness and stability you developed are in fact necessary, and foundational practices are often given in many Zen lines to develop exactly those qualities.
Astus wrote:Since the nature of mind is already empty and aware, there is nothing to develop or attain, only to realise this fact for yourself. That's how Zen has no real stages. But, it is still possible to devise some levels, like using the 10 bodhisattva stages, or the whole 52 stages system, or as Ven. Shengyan summarised it as (1) calm mind, (2) unified mind, (3) and no mind.
While "to realise this fact for yourself" is the point of Zen practice, it's important to point out that "realise" in the sense of the fruition of Zen practice means "integrate, embody, bring all activities of body/speech/mind into accord with". It is possible for a practitioner to come to that full fruition all at once. But since this is not the case for almost everyone, again, I think it important to stress that Zen does indeed have defined stages and progress...even if the insight into one's nature which is the gate and basis of Zen practice is not something new one "gains", and so is not something progressively attained.
So we always end up talking about levels, real stages, development and attainment. As Astus points out, there are various formulations of these stages. Whichever of them one uses (according to the tradition of one's teacher, I presume) is not so important: the important thing is not to stop halfway through, and I think all traditions of Zen would agree on that. Sheng-yen's three stages of calming mind, unifying mind and no-mind are a description of the approach to the initial entering into the "realm of Ch'an", or seeing one's nature...not the entire course of practice. Thus he will also say: "Nevertheless, a newly enlightened person who has just entered the realm of Ch’an is still at the starting section of the entire passage of Ch’an. He is like one who has just had his first sip of port. He knows its taste now, but the wine will not remain in his mouth forever. The purpose of Ch’an is not just to let you take one sip, but to have your entire life merge with and dissolve in the wine, even, to the point that you forget the existence of yourself and the wine."
At least on the Japanese Rinzai side of things, the understanding of stages and progress along the path is shared in a unified manner from line to line. If you have any interest in that, check out the text I mentioned from Torei for a good overview of how it's concretely applied in practice.