Plain white salt blocks are safe and palatable for both cattle and horses. The classic red trace mineral block formulated for cattle is not dangerous for horses; it just doesn't have enough of the trace minerals to balance a forage-only diet in a horse. There are, however, many other types of mineral blocks that are made for cattle that are not suitable for horses (e.g., medicated, protein, phosphorus, or sulfur blocks, to name a few).
If a mineral block is needed to balance the nutrition of forage-only diets for cattle and horses, it is going to be difficult to find one that suits both species because they have different requirements. However, if the horse is getting the mineral nutrition from somewhere else in the diet (e.g., a concentrated feed), then offering either the white, natural salt rocks, or trace mineral red salt blocks is fine.
If you have any question about the suitability of a product for horses, contact the manufacturer for further information. If a manufacturer is unable to tell you if a block is safe for horses, it would be best to find another product.
For more equine nutrition information visit ker.com.