In principle, ABS is safe for contact with (cold or room-temperature) food. The two main concerns specific to 3D printing are, assuming you start with a filament that is not itself contaminated:
Pores and holes in the printed part which may harbor bacteria
Impurities introduced into the plastic during the printing process
I doubt that the silicone mold will capture the pores and holes with sufficient detail to be of any concern (it certainly won't capture the internal structure, only the surface).
That leaves us with 2. It has been noted that brass nozzles contain trace amounts of lead. This lead can contaminate the printed part, which may in turn contaminate your mold, which may in turn contaminate your food. I don't think this is of realistic concern, since we're looking at trace amounts of trace amounts of lead. The nozzle might also have burnt plastic stuck to it (which might be carcinogenic) so you should make sure to do the print with a very clean nozzle and at a temperature that is not too high.
ABS is food safe for contact with cold or room-temperature food. It is however not food safe for contact with hot food, because at higher temperatures the food may leach certain chemicals out of the plastic. Your application is one of low temperature, but silicone is not food and might perhaps leach some contaminants out of the plastic, regardless of temperature. However, this concern is not specific to 3D printing, as it applies to the method of making moulds out of Lego bricks as well. Therefore, making moulds from 3D printed positives does not appear to be different in a food safety perspective from making them out of LEGO blocks.