block1.js

const observer = new MutationObserver(mutations => {
    mutations.forEach(({ addedNodes }) => {
        addedNodes.forEach(node => {
            // For each added script tag
            if(node.nodeType === 1 && node.tagName === 'SCRIPT') {
                const src = node.src || ''
                const type = node.type
                // If the src is inside your blacklist
                if(needsToBeBlacklisted(src, type)) {
                   // Do some stuff that will prevent the script tag loading ;)
                   // (See below…)
                }
            }
        })
    })
})

// Starts the monitoring
observer.observe(document.documentElement, {
    childList: true,
    subtree: true
})