The director of Belle describes how she came to make the film in this article at the Independent: Director Amma Asante on the inspiration behind her film Belle:
The historical facts such as the painting, the period, and her upbringing within a privileged family that raised her as an aristocrat, compelled me to define my movie as one about race, politics, art and history - but also one that looks at themes of gender, identity, belonging and equality. Being bi-cultural - I was born and raised in London, the child of West African immigrants - I feel at the intersection of all these themes and wanted to place Belle there, too. One element I wanted to explore was the issue of “ownership”. Although the slave's experience was entirely different to that of the woman, both were nevertheless “owned” - the woman being her husband's property, assuming his last name on marriage.The trailer does not, in my view, give an effective grasp of the film, though it gives some impression of the locations and filming. The review at SIGNIS is more useful in giving some idea of what the film is about - though it commits a quite striking gaffe in omitting the name of the lead actress, Gugu Mbatha-Raw, at the head of the review. The comparison to the novels of Jane Austen is quite justified, with an elegance and style in dialogue being present throughout Belle, something that I found very attractive in the film. There is also a "question and answer" session with the director and some of the cast at the Toronto Film Festival 2013, during which the director says more about how the film came to be made. Do look out for the contributions of Tom Wilkinson, Gugu-Mbatha-Raw and Emily Watson in this session (starting at 7:10). Listen to these contributions before you read the next paragraphs.
There is a clear historical context to Belle - the strict ordering of the aristocratic society of the time, the slave trade and the case of the slave ship Zong (the most careful source I can find on this is here, though I do not have the historical expertise to authoritatively judge it) and the life story of Dido Belle herself. However, in the same way that the film Philomena has a representative character, Belle explores a range of themes in its narrative, in a kind of representative way. As you watch the film, you can recognise an exploration of the place of women in society (both in the particular context of the 18th century and in the context of marriage such as would still be relevant today), the relationship of rule or order in society to the wishes of the individual (scandal), the question of how a person is to be valued (title, wealth, family) and most prominently the question of racial discrimination.
I felt there was a subtlety hidden in the way in which the film treated the theme of marriage, though the director does not identify marriage in itself as one of the themes she wishes to explore. In the first place, Belle, at least in the film, is the illegitimate daughter of one of the sons of the family. There is then the question of marriage seen as an institution at the service of wealth or status in society - these are the drivers that lead to marriage rather than the affection of one person for another. And then - shown in the closing scene of the film - there is the love of one person for another, over-riding any considerations of status or wealth, as the key point leading to marriage. (This is compromised, though - if you watch this scene carefully you will see what I mean.) The subtlety lies in how marriage is to be perceived as something pertaining to both an objective institution and arising from an inter-personal affection. The film seems to suggest it is a question of either/or. Gugu-Mbatha-Raw's observation in the Toronto Film Festival "question and answer" that the theme of the film is about being oneself rather than letting oneself be defined by others has some truth to it; but, in the end, I do not think that even the film itself suggests that being oneself is a complete and adequate basis for life decisions.
From a Catholic point of view, the film's portrayal of the institutional dimension to marriage lacks the key components - communion of life for the birth and upbringing of children. Perhaps the film's delineation of how marriage was viewed in 18th century aristocratic society is an accurate one; but the unfortunate aspect of this is that the film too readily suggests replacing that (in itself deficient) view with a view based only on the subjective attraction between people. It misses the point that there is another institutional dimension to marriage.
I suspect that most people who see this film will just recognise it as a beautiful film and enjoy it as such.