Are American food colourings weaker than in other countries?
The simple answer is: No.

First of all the recipe says 1 tablespoon of blue food coloring. This is pretty typical for a velvet cake. In fact it's actually less than I would expect. This is because the original color is dark brown from the cocoa powder. To cover that up and turn it into a different color requires a lot of food coloring. 

If you take a look at these standard red velvet cakes, you will see that they ask for 1 ounce food coloring. And 1 fluid ounce is equivalent to 2 tablespoons. 


http://allrecipes.com/recipe/red-velvet-cupcakes-3/detail.aspx
http://allrecipes.com/recipe/red-velvet-cake-vi/detail.aspx
http://allrecipes.com/recipe/red-velvet-cake-iii/detail.aspx