How do I make blocks one directional?

You can use Dot Products to see if a part is seen in another part's field of view. It basically returns 0.5 to 1 if the part is in front of another part and returns 0 or less if it's in the side/back. Here's a YouTube tutorial if you want.

local part1 = workspace.Part1
local part2 = workspace.Part2

while true do
    task.wait(1)
    local partToPart = (part2.Position - part1.Position).Unit

    local dotProduct = partToPart:Dot(part1.CFrame.LookVector)

    if dotProduct >= 0.5 then -- if part is within the part1's field of view
        print("Part2 is within Part1's FOV")
    else
        print("Part2 cannot be seen in Part1's FOV")
    end
end

Use raycasting.

local part1 = workspace.Part1
local part2 = workspace.Part2

local params = RaycastParams.new()
params.FilterType = Enum.FilterType.Blacklist
params.FilterDescendantsInstances = {part1}

local length = 50

while true do
    local hit = workspace:Raycast(part1.Position, part1.CFrame.LookVector * length)
    if hit and hit.Instance then
    --do code
    end
    task.wait(0.2)
end