My aim as a landlord is always to try and keep my house from sliding into looking like 'a renter' and getting lower and lower rent each time because it starts looking shabby. So, we aim to try and freshen the paintwork between tenants anyway (one reason all the rooms in the house are the same colour). Maybe your landlord has this in mind and will accept a few blu tack marks as fair wear and tear - you won't know until you ask. Blu tack marks wouldn't pee me off as a landlord; huge divots in walls or drilled holes with rawl plugs make me mad as hell.
Tot up how many marks in how many rooms and if they won't come off, fess up to the landlord while you are still there and can do something about them. If the landlord won't accept them as fair wear and tear and he/she wants to charge you for redecoration, offer (in writing) to buy a grease-sealing undercoat plus the correct paint and promise to be scrupulous about covering floors and furniture, and taping any glosswork to protect it, and say you will paint the wall. The whole wall so that it matches in. Also take photographs of the marks and the whole room so that there's no doubt about the extent of the marks - that you haven't destroyed the room, you've only left a couple of marks on a wall.
If your landlord has put your deposit into a protection scheme (which they should have), then any dispute over your deposit has to be decided by an independent adjudicator before any of the deposit goes to either of you. You can raise the dispute with the scheme even before you leave if your landlord is threatening to dock it. The fact that you've offered in writing to make good the marks at your own cost and in a responsible way will go a long way with the adjudicator, who is usually on the side of the tenant, and should ensure you get your deposit back.