You will have to do the train journey on 2 different trains.
First catch the Virgin Train from London Euston to Preston, then change and get on to a train from Preston to Blackpool.
The timing is roughly between 3hrs 10mins and 3.5hrs.
You havent stated a travelling time so it is difficult to get an exact travelling time due to connections.
The average seems to be about 3hrs.15mins with one change.
I dont no sam but if you visit http://www.wesearch4you.co.uk/ go to main menu torist info theres a train transport timetable there times of trains ect hope this helps good look ,me and the wife are going to blackpool next weekend ,seeya
6 minutes and 4 seconds