In admitting with Kant that the beautiful is that which pleases universally without concepts or that when one makes a judgment only in accord with concepts every representation of beauty disappears, can one maintain inversely that the ugly is that which displeases universally without concepts? In other words, how can there be established, according to you, an argued discussion on art and the aesthetic, how can we conceive from the point of view of the critique of the aesthetic judgment, the paradoxical tension between the universal and the singular?

I believe that to clarify the question and direct the answer, we must situate ourselves in the work of the Universal, because here we have a Universal that Kant, at the beginning of the third Critique,  opposes to the Universal of the determinative judgment.  This latter posits the rule, and experience is subsumed under it; the case is therefore placed under the rule.  The inverse   situation is in this sense exceptional and incredibly disconcerting (deroutante). It is the case of the aesthetic judgment; here all the judgments are singular, but precisely singular, not by way of subsumption, but by direct apprehension. The hermeneutics of Gadamer allows us to give full force in my view to this initial Kantian position of the singularity of the aesthetic judgment: This rose is beautiful.  It is a singularity that includes the idea of a hold on us by the beautiful thing. To a certain degree, the idea of a hold marks a certain rupture with Kant as to the mode of comprehension, of the apprehension of the singularity.  But what continues to effect the force of the Kantian analysis is that there is nevertheless the universal.  With all his might Kant resists the idea that one cannot discuss colors and tastes, which would isolate each of us in his pleasure, in his mood. Now, how can there be a universal?  The great force of the Kantian solution is to have staked everything on the idea of communicability. Communicability is the modality of the universal without concepts; it is a matter of a powder train, of contagion from one case to another.  And what is thus communicated? It is not the rule, nor the case, but the game between understanding and imagination.  Each of us relives this kind of debate, of conflict, between a rule and the imagination, which, in the sublime is found to be affected by overflowing, by the excess of the object over the capacity to include it, whereas in the beautiful there is an imagination of harmony.  It is this contamination, this powder trail, which involves subjects in communion, in participation in the same emotion. 

Put differently, do you reject the aesthetic relativism which one might maintain, for example, from an ethnological or anthropological point of view, both in time and in space?    

At first glance, one might say that sociology shows that Kant is wrong, because there is a historicity which in no way shows up in his analysis; in fact, in a first analysis, the history of styles and tastes demonstrates that he is wrong.  In a second analysis, however, this latter proves him right, because in the long view, as this would appear in Malraux’s works, there is revealed a dimension of transhistoricity.  And this transhistoricity consists in sum in the permanence, or better the perdurance, of works of art in escaping the history of their constitution.  What is striking in the aesthetic experience is that, in differentiation from economic and political phenomena where the result is in some way proportionate to its production; the result here is in excess of its production.  One could say that the work of art escapes the history of its constitution,  and it is this temporality of a second degree which constitutes the temporality of communicability.  This transhistorical communicability is the rational equivalent of objectivity, as much in the beautiful as in the sublime.  To continue in this direction would require analysing the specific temporality of the work of art, which Kant has not done. 

Which Heidegger has done . . .

Which Heidegger has done indeed; and with him the whole hermeneutic tradition, because this has been confronted in a much more menacing way than Kant was by historicism, by historical relativism.  Thus it is that the reconquest of the transhistorical over the historical constitutes the post-Kantian benefit of a return to the Kantian aesthetic.  We can reflect on the curious status of the work of art, which has perhaps an equivalent in the speculation over angels and their temporality, which is not the immutable temporality of God, nor the precariousness of human things.  The medievals had forged, to this effect, the concept of the perennial, of the sempiternal. 
Here there is more than an approximation but a kind of profound kinship with the status of angels in the great medieval tradition -- but one which is also multi-secular -- and the idea of a species with one single individual.  And in short the work of art is a species with one single individual.