Multi-access edge computing (MEC) is a keystone for enabling wide range of vertical applications with diverse quality of service (QoS) requirements over 5G network. With the roll-out of 5G networks across the globe, the mobile network operators (MNOs) are looking forward to generating business-to-business (B2B) revenue by provisioning edge cloud on their networks and hosting the applications of the 3rd party application service providers (ASPs). However, in order to accelerate the adoption of MEC, it is essential to adopt open and standardized service platform as well as a flexible and trustworthy framework for service level agreement (SLA) enforcement. Edge service provisioning will involve strict QoS guarantees for offered edge services based on heterogeneous QoS requirements of different applications, thereby requiring robust, flexible and credible SLA verification and charging as a part of business support system (BSS) of the MNO. Conventional cloud SLAs are not suitable due to lack of the flexibility and credibility required for automatic enforcement in a dynamic and heterogeneous environment. To address this challenge, we propose a blockchain-based framework for credible SLA enforcement. The proposed framework leverages smart contracts to provide an immutable solution, and ensures credibility by introducing an auditing mechanism for verifying the SLA violations.