Interesting line of thought, and I've been asked a similar question by Green Belts before when I've taught in training programmes. The question in my case was: How do we ensure that people certified in a 6 Sigma course in one company are using the tools and methods of 6 Sigma as well as in some other organization. The answer, would, of course depend on many factors. Some things to consider:
6 Sigma projects are always team activities and involve the skills that various team members bring to the table. Some of these skills are - data analysis, subject matter expertise, leadership and coordination skills, ability to think creatively and out of the box, etc., which all become relevant in different parts of the same 6 Sigma project
Some tools are commonly used across diverse 6 Sigma implementation. Constructing a project charter, process maps, sample size calculations, hypothesis testing, RCA using Fishbone diagrams, CE matrices, FMEA, DOE, Pugh Matrices for controlled convergence and control charts are some of the universally used tools. In addition, there may be project-specific tools and heuristics, which a lot of companies develop depending on their processes and systems. Some of these other tools could be QFD, data mining approaches, robust optimization algorithms, Monte Carlo and other methods of simulation, etc
There is no gold standard that says that this is how things ought to be done, when it comes to, say, process maps, or fishbone diagrams. With methods like hypothesis testing, which have been well developed in statistics, there is more certainty on what can be done with data and what kinds of tests reveal what things about the data. However, good judgment is needed whenever we use heuristic tools or subjective tools like CE matrices, fishbone diagrams or Pugh matrices. This is because by nature, they're team activities, which require the expertise of subject matter experts to reveal and analyze root causes, or validate solutions.
As Vishvadeep has pointed out in his post above there are standards like ISO 31000 for risk management, and 31010 additionally. Similarly, there are standards and best practices like eSCM, ITIL and so on which address different aspects of organizational systems. These can be taken as cues for how to construct systems, for benchmarking, setting up measurement systems, and so on. However, the standards in themselves don't provide numerical certainty that your approach is "consistent".
The creativity inherent in 6 Sigma projects and application of tools/methods is what separates the wheat from the chaff in 6 Sigma. You'll agree that a lot of the emphasis on tools and methods given in BSS courses are focused on these aspects. Creativity can be learned, however, especially using TRIZ. Methods like that can be used for solutions
I heard a quote once by a colleague - that Black Belt should embrace change also, and not only certainty. Certainty is great for ensuring results get realized. But Black Belts (and Six Sigma professional in general) are change agents. If someone else can make the same difference we can to the organization, we become irrelevant. Creativity is the driving force for this differentiation, in my view. So, unless we're creative, regardless of how much we understand about 6 Sigma and how persistent we are about bringing change, we may not be able to apply these methods in organizations successfully.