Does fish blood have a smell, how to remove it?
truly fresh fish should not smell of anything except sea water


While that might be true of most fresh fish fillets, fish guts most certainly smell, and unless all of those little guys have been opened and cleaned, you're almost certainly tasting/smelling the guts. The big fish in the case are nice and clean because they're gutted and washed. Often, smaller fish like Sardines are sold whole without gutting and washing. This can be kind of a dicey proposition if your store doesn't do a ton of volume, or you're not super close to the port at which they were caught.

Are you removing the guts and washing them before you cook them? That would be the first step to reduce the flavors you don't like.

Is the thin flesh of the belly deteriorated, perhaps even with the guts falling out? This is called belly burn, which occurs when the guts were left in too long and start to digest the fish from the inside out. These fish should be discarded (or used as bait if you decide to go DIY)