I have grouped these two places togeather for convienience. There are three places to fish at Blyth, the docks, the pier, and the Beach. The docks are a good spot on those rainy days because you can fish from the comfort of your car. Virtually anywhere is worth a try and often at weekends you will have to make do with anywhere. But if you can fish near the life boat station. There is a good depth of water everywhere and the bottom is fairly clean. Strong currents particulary during spring tides call for the use of grip leads to avoid tangles/crossovers.
Blyth pier is accessed by parking at the boat/caravan yard at South Beach and taking a walk northwards (just follow the other anglers past the "private road sign"). Normal pier fishing techniques apply but use a strong rod and preferably a mulitiplier as there is a long drop to the sea. The section after the bend gives the better catches and but is nearly always crowded, The water is deeper here with a rock and kelp bottom. Before the bend the bottoms cleaner, there are a few snags close in, but even a short lob will avoid trouble on the seaward side. Both sides fish well at the end so take two rods if you can, a long cast is best if you fish into the mouth. At low tide try crossing the sand under the pier and casting into the river, this is a good spot when other places are unfishable or the pier too crowded. There are some awkward rocks between the beach and water, access is easier near the wall. For mackerel in the summer go through the gate at the end and fish straight out. Best to go early morning to have any chance of a place.
Blyth Beach more commonly known as South Beach is a low tide mark and is best fished at night with a sea on (then again where isn't). At high tide cast out to one of the holes that regularly open up, a rolling leger helps locate them. This is also a popular area for bathers and surfers so have another mark in mind if the weather is warm. Park in the car park at Link house, you get a good view standing on the promanade so spend a few minutes here reading the beach. The stone pipe provides a good fishing platform but for safty best avoided during heavy seas, letting your bait get swept by the current against it is worth a try as any fish swimming along with the tide will have to navigate around it(night only for whiting and dabs). WARNING Blyth has a bad reputation as the drugs capital of the UK so when asking directions to the local dealer make sure you specify it's a tackle dealer your after. while I have never had any problems it is proberly not the best place to fish alone. Half way between Blyth an Seaton sluice there is a car park, the beach opposite fishes well on the ebb with a large gulley running along the lowtide mark towards the farm. As you travel South towards Seaton Sluice the beach above the low tide mark starts to steepen and holes often open up at the base of this slope which are worth trying at high tide, about two hours either side no more other wise you will be casting in shallow water (although I've caught whiting and flatties in only a few feet at night). Park in the carpark next to the roundabout at Seaton Sluice and fish opposite leaving the hole 50 yards to the South free for me during the winter months (vandalism is a problem in this carpark). I like to fish two rods here, one casted out as far a possible and another a short lob into one of the holes. I like to fish the holes with light tackle as they are often filled with dabs and whiting which keep you occupied while you pray for the big one. Seaton Sluice also fishes well at low tide next to the sluice mouth and the rocks are worth a go, park next to the pub and walk over bridge and down the steps onto the rocks. In general Blyth-Seaton sluice is a good choice for anglers who don't like losing half their tackle but are prepared to wear out shoe leather at low tide to see which holes have opened up. As a end note Seaton Sluice beach was a popular place to burn witches in the middle ages. Any large fires you see nowadays while fishing is probably some local kids torching your car