Add this to @types/@sanity folder in the root of your TS project.

block-content-to-react.d.ts

declare module "@sanity/block-content-to-react"
 {
  import * as React from "react"

  export interface BlockContentProps {
    /**
     * Pass in either an array or a single object of [Portable Text](https://github.com/portabletext/portabletext)
     *
     * *This is the only required prop*
     */
    blocks: any[] | any,
    /**
     * When more than one block is given, a container node has to be created. Passing a className will pass it on to the container.
     * @note see `renderContainerOnSingleChild`
     */
    className?: string
    /**
     * When a single block is given as input, the default behavior is to not render any container.
     * If you always want to render the container, pass `true`.
     */
    renderContainerOnSingleChild?: boolean
    /**
     *  Define custom serializers
     *
     */
    serializers?: {
      /**
       * Serializers for block types
       * @example
       * ```jsx
       * const input = [{
       *   _type: 'block',
       *   children: [{
       *     _key: 'a1ph4',
       *     _type: 'span',
       *     marks: ['s0m3k3y'],
       *     text: 'Sanity'
       *   }],
       *   markDefs: [{
       *     _key: 's0m3k3y',
       *     _type: 'highlight',
       *     color: '#E4FC5B'
       *   }]
       * }]
       *
       * const highlight = props => {
       *   return (
       *     <span style={{backgroundColor: props.mark.color}}>
       *       {props.children}
       *     </span>
       *   )
       * }
       *
       * <BlockContent
       *   blocks={input}
       *   serializers={{marks: {highlight}}}
       * />
       * ```
       */
      types: Record<string, (props: any) => JSX.Element | null>
      /**
       * Serializers for marks - data that annotates a text child of a block.
       * @example
       * ```jsx
       * const input = [{
       *   _type: 'block',
       *   children: [{
       *     _key: 'a1ph4',
       *     _type: 'span',
       *     marks: ['s0m3k3y'],
       *     text: 'Sanity'
       *   }],
       *   markDefs: [{
       *     _key: 's0m3k3y',
       *     _type: 'highlight',
       *     color: '#E4FC5B'
       *   }]
       * }]
       *
       * const highlight = props => {
       *   return (
       *     <span style={{backgroundColor: props.mark.color}}>
       *       {props.children}
       *     </span>
       *   )
       * }
       *
       * <BlockContent
       *   blocks={input}
       *   serializers={{marks: {highlight}}}
       * />
       * ```
       */
      marks: Record<string, (props: any) => JSX.Element | null>
      /** React component to use when rendering a list node */
      list: React.Component
      /** React component to use when rendering a list item node */
      listItem: React.Component
      /**
       * React component to use when transforming newline characters
       * to a hard break (<br/> by default, pass false to render newline character)
       */
      hardBreak: React.Component
      /** Serializer for the container wrapping the blocks */
      container: React.Component
    }
    /**
     * When encountering image blocks,
     * this defines which query parameters to apply in order to control size/crop mode etc.
     */
    imageOptions?: any
    /** The ID of your Sanity project. */
    projectId?: string
    /** Name of the Sanity dataset containing the document that is being rendered. */
    dataset?: string
  }

  /** React component for transforming Sanity block content to React components */
  export default function BlockContent(props: BlockContentProps): JSX.Element
}