The blood seems to come directly through the cracks of the engraving itself, and at the same time, Jekyll's own hand begins to drip.

"Ah- yes, sorry, don't worry, it will only do that if you touch that part! I said I would find some way to give you command over me, this functions much the same as a Command Seal, but, well, it was necessary to create a direct connection..."

In other words, he's sealed a piece of his soul inside, it's the reaction to that raw bit being prodded.