General- Avoid anything bottom dwelling, small, slow, diurnal (you'd want to pick up something nocturnal or crepuscular, so that it wouldn't be sleeping when they crayfish was awake), aggressive or that would grow large and then eat your crayfish.
Species specific- "Blue Lobster" encompasses a large range of species that are commonly caught/bred/sold. So without knowing what species it is, you'd really have to defer to general advisory on crayfish. If you could find out what species it is you could better choose fish, here's some examples:
For Procambarus alleni - Best kept in a species only tank. Highly aggressive and carnivorous (will often, but not always, actively hunt any type of tankmate).
For Cherax albidus (previously known as destructor) - Best kept in a species only tank. Reaches an impressive size/weight and can be highly aggressive.
For Cherax quadricarinatus - Best kept in a species only tank as a juvenile (due to them eating mostly protein as juveniles, adults prefer higher amounts of vegetable matter), but will be fine with most fish (larger and smaller) as a young adult - adult. This is a rather docile species of crayfish which is often easily kept in small groups and with a large range of tankmates.
For Cherax cainii - Best kept in a species only tank. Though, I've seen them kept with larger catfish, cichlids, etc without much damage or loss to the fish. This is still not recommended though, as both the catfish and cichlids could kill the crayfish when it molts.
There are also any number of additional "morph" crayfish, which can be sold as "blue lobsters". Some of these are aggressive, some of them not. For example, my blue Marmorkreb is easily housed with other Marmorkrebs, shrimp, fish (loaches & juvenile mollies) and snails. My blue C. shuffeldtii are not so easily housed with each other crays, smaller shrimp and snails but do fine with tetras and small plecos (they seem to ignore the fish). Etc, etc, etc.