I'll repeat that your statutory rights are with the retailer, not the manufacturer. (If you buy a telly which then explodes and burns your house down, it's the retailer who, in the first instance is held responsible, not the manufacturer. It would be up to the retailer to then claim against the manufacturer).
However most food production companies in the UK take their responsibilities seriously, so you'd probably get a positive response if you wrote to the manufacturer.
Blue colourants within the food production process are usually used to indicate to indicate that the end of a particular production run is imminent. (It's similar to he way that till rolls have a pink banding to warn checkout operators that they need to change the till roll). The colourants are completely harmless but blue-dyed products aren't normally allowed to reach the packaging stage. So there would be little point complaining to 'Health & Safety' officials, as blue-dyed pork crackling is no less safe than the regular product (and perfectly legal to sell).
Chris