That appears to me to be the result of citrus whitefly larva, which are not only tiny but also almost transparent (hard to see without some sort of magnification).
These larva attach themselves to the underside of citrus leaves and flower stems, they secrete a "honeydew" like substance and this is perfect food for sooty mold which can be found on flowers and leaves.
In your photo there are enough of these larva present that I can see them on the undersides of the leaf just below the lowest dead flower, it is probable that the larva did some migration to the flower stems since those would have more sap flowing to them than the leaves.
If you shake that tree and tiny white flies start fluttering around the tree, you have discovered the problem causer.
Citrus trees are notorious Nitrogen feeders so, to help the tree be healthy adding rhizobacteria so it can get to the root system is one of the best things you can do for any citrus or other fruit trees.
A spray down with a good aerated compost tea will also help the tree fight off any new attacks or diseases.
Since the tree is container bound it would be a good idea to lift it every two years and give it a root trimming so that there is room for root growth in the barrel half. (think bonsai root trimming)
container grown citrus can use up their soil nutrients in a single year when artificial fertilizers are being used.