Flame retardant polymeric blend compositions including a synergistic combination of flame retardants, including a halogenated flame retardant and a nanoparticulate, such as an organically modified clay or a natural nanoclay. The compositions are thermoplastic elastomers, include a polyolefin and a styrenic block copolymer, and exhibit high char formation upon burning as well as desirable vertical burn characteristics. In a preferred embodiment, the flame retardants include at least a halogenated flame retardant and a metal oxide.