A black hole is little more than a spatially closed, gravitationally bound quantity of matter with an escape velocity greater than or equal to the speed of light, but for a black hole to have a validated existence, it must be perceived from OUTSIDE, not inside. In a universe-sized black hole, it would not seem like a black hole from within. The mass inside the black hole would equal the mass necessary to render spacetime flat, and if more matter fell in, the radius would increase proportionately, and the ratio would remain constant.
The real issue is whether or not our calculations of the mass of the universe are accurate; it is most certainly not so. Dark matter, never seen, never measured but only postulated from gravitational expectations that were not met by current models suggest one mass, conventional empirical (measured, not inferred) evidence suggests a much smaller mass. Which is correct? If history is a reliable guide, neither one.
It is essentially irrelevant if we are or are not in a black hole. If we can leave our universe, where would we go? Currently unanswerable without an external frame of reference, which would be a Catch-22.
Our meager understanding of gravity (we still do not know what it is, only how it behaves locally) is insufficient to answer any of these questions. Speculating on an answer is indulging in intellectual mythology. Without better data, we are just making up solutions to a question we do not fully understand, and that is not only bad science, it is lousy thinking.
And for the record, math as a form of symbolic logic is devious - it can be used to support or decry, but unless it is used as a proof, it is only distracting and often misleading. Formulae are inherently limited, and as such can be rightfully accused of cherry-picking the data to only use terms that support the hypothesis. Math cannot suggest or imply a solution any more than a hammer can recommend house design. People anthropomorphize math too frequently, when in fact, it is people who suggest, and people who infer. And frequently, they do so with poor judgement until enough data is available to make the conclusion self-evident.
We have not proven that the universe is expanding. We have not validated the mass of the known universe. We have not even accurately measured the distances to the most distant objects yet observed. All we have done is observed motions on a galactic scale, and applied gravitational formulae that were derived locally with an infantile understanding of gravity itself. Hubble accurately measured red shifts of distant objects, but in his 1942 paper in the Sigma Xi Journal questioned the relation between his observations, actual motion and an expanding universe. If he had the courage to question the very thing that gave him fame, we should have the courage to honor his skepticism and wait until we have an ANSWER and not a guess born out of impatience and hubris.