the easiest way is from london euston on virgin. direct to preston and then a local train (northern trains or transpennine express) on to blackpool. (blackpool has 2 stations, north and south, north being the main and central one). travelling time in total approx 3 hours depending on transfer time at preston. check thetrainline.com for train times and dates.
I would recommend looking at the journey planner onhttp://www.transportdirect.info
as times vary according to when you want to go.
This site gives bus, train and car planners, it includes the route, trains to catch and even the buses.
Usually 3-4 hours would be normal