Common liquid transportation fuels are composed of a variety of large hydrocarbons, some of which are quite reactive at low temperatures. While it is known that pure fuels with strong low-temperature chemistry can produce dramatic changes in the turbulent flame structure and burning characteristics, it is unclear how a blend of fuels with different reactivities would influence turbulent combustion. This study seeks to answer this question through the experimental measurement of turbulent premixed flames of blends of n-octane and iso-octane. Experiments are performed in a reactor-assisted turbulent slot burner at two different preheat temperatures. At a preheat temperature of 450 K, the two fuels and their blends behave similarly in terms of the turbulent flame structure and burning velocity. However, when the preheat temperature is increased to 700 K, the blends with large amounts of n-octane transition into a low-temperature ignition regime in which the pre-flame structure is drastically modified. Formaldehyde laser-induced fluorescence measurements reveal significant formaldehyde production in the bulk flow exiting the burner, which leads to noticeable changes in the flame structure and a slight increase in the turbulent burning velocity. Furthermore, this regime change is not seen for the blends composed primarily of iso-octane, confirming the role of low-temperature reactivity in producing changes in the turbulent flame.