Using anything from a simple vinegar and water solution or a mild dish soap along with warm water can make all the difference. You can also use your regular multi-purpose dishwashing liquid, albeit diluted with water. Although, you cannot bleach off using the harsh chemicals away on these blocks as it will cause wood to dry out.
Here are a few steps that you have to follow -
• Take a bucket with water and then use it to dip the dirty blocks.
• Once you scrub the block, then immediately dip in another vessel that has clean water
• You can then wipe these clean using a dry cotton cloth
• Place these under shade to dry completely.