We studied magnetoconductance (MC) and magnetoelectroluminescence (MEL) in organic diodes from blends of pi-conjugated polymers and fullerenes at various concentrations, c. The MC response is composed of several components that depend on the applied bias voltage and c. A dominant positive low-field (LF) component, which also governs the MEL response, dramatically decreases and broadens in the blends, thus unraveling a positive high-field and negative LF components. The positive MC components are caused by electrostatically bound e-h polaron pairs in unblended devices, and charge transfer pairs in the blends, which are dominated by two different field-induced spin sublevel mixing mechanisms. In contrast, the negative LF response is due to e-e and h-h pairs; this is confirmed by studying MC in electron- and hole-unipolar devices, which lack positive MC response.